@SnapGeeko I find it useful to check specifications in the User Guides / Owner's Manuals when the product pages don't provide any clear indication of differences. But fyi, the U Series (UltraSharp) is Dell's top-end business display category (with the UP UltraSharp PremierColor being sort of a specialized line), and the P Series (Professional) is the next step down. And that shows, because in this case the "2020 model year" U Series has some advantages over the "2021 model year" P Series display:
U2520D:
Supports HDR (meets VESA DisplayHDR 400 standards)
Supports DisplayPort 1.4 input. Allows more input bandwidth, which can be useful for daisy chaining
Supports HDMI 2.0 (related to HDR)
USB-C input can provide up to 90W to attached system
3x USB-A ports and 1x USB-C port (the latter is a downstream port for a peripheral, not an additional USB-C input)
P2421
No HDR support
Only supports DisplayPort 1.2
Only supports HDMI 1.4 (not really a limitation given that that's fine for QHD, and HDMI can't be used in a daisy chain)
USB-C input can only provide 65W to attached system
4x USB-A downstream ports. No downstream USB-C port.
I got all that from spending about 5 minutes looking at the manuals of each display.
Fyi though if you want QHD resolution I'd really recommend stepping up to 27". If you look at the dimensions of each display, you'll find that they're not very much larger, but the extra space brings the pixel density down just enough that most people can use QHD without enabling any display scaling, which some Windows applications still don't handle very well. But QHD on a 24-25" display would be tough to use without scaling. Or if you want a really sharp 24" display, then go all the way to 4K. I honestly don't understand why 24-25" QHD displays exist, but I guess I also don't understand why the similarly positioned 27" 4K displays exist either, except I guess to satisfy the market craze for 4K. But a 27" 4K display's pixel density is high enough that you have to enable scaling with all of the potential issues that creates, but not high enough to create that really sharp "Retina display" press quality text experience -- so it's sort of the worst of both worlds. A 25" QHD seems like it would be in a similar position.
Curious, when you mentioned "display would be tough to use without scaling". What does it really mean? Isn't having less size and more resolution a good thing for sharp view? For ex: Macbook pro is 15'' and > QHD resolution and text and everything looks so sharp.
@SnapGeeko I forgot you were on a Mac. What I said applies more to Windows since macOS handles scaling a bit better overall, but even there it's generally better to use even multiples of scaling like 1x, 2x, or 3x, rather than 1.25x, 1.5x, and so on -- which is likely what you'd use on a 25" QHD display. Bear with me here. macOS going back quite a long while was designed with a "reference" display standard of 100 pixels per inch. Windows used 96 ppi. That meant that if you were using a display with the reference pixel density of that OS, then something that was meant to appear 1" wide in the real world -- like the ruler in Microsoft Word, for example -- would actually be 1" wide on that display. And if you look at lots of Apple's pre-Retina displays, like the 23" 1920x1200 Cinema display and the 30" 2560x1600 display -- you will find that they had 100 ppi densities. (They went a bit higher on laptops because laptops are designed for a shorter viewing distance.)
Then Retina displays came along, where Apple basically doubled the "reference" pixel density in both axes. So a display that used to be 1440x900 would become 2880x1800, for example. And to help developers scale their artwork to display properly on those displays, macOS allowed applications to bundle so-called "@2x" versions of their art assets, so that if it was running on a Retina display, it would load those assets rather than just trying to scale up regular art assets. And if you were using a display that has a pixel density of about 200 ppi, then the on-screen content would remain a sensible physical size on the display, AND that "@2x" art would be pixel perfect on your Retina display because it would have been designed with that specific 200 ppi density in mind. A 24" 4K display has a pixel density of 183 ppi, for example -- because it's very close to taking that older 23" 1920x1200 display I mentioned, with its 100 ppi density, and just doubling the pixel density on both axes while maintaining almost the same size. Technically things would be a tiny bit larger than intended if you used the "@2x" art with its default scaling, but it's close enough, and slightly larger usually isn't a problem.
But a 25" QHD display is 117 ppi. That's not a whole lot higher than "reference", but it's probably enough of a step up from reference that if you didn't enable scaling, things would be just a bit too small. But if you DO enable scaling, you wouldn't be able to use 200% scaling in order to use the sharper "@2x" assets, because 200% scaling would make things way too large and completely eliminating the larger workspace benefit of higher resolution. You'll instead probably have to choose a mode that would be something like 125% scaling. But when you do THAT, then none of the art assets available to macOS will be pixel perfect on your display bercause none of them will have been designed with that scale factor in mind. Instead, macOS will use the regular art assets and then use GPU scaling to blow them up a bit. That won't look as good as having a display with a pixel density that allows art assets to be used without any post-render scaling.
You can sort of see the impact of this on the Retina display built into your Mac. The last 15" MBP had a display resolution of 2880x1800, with a 220 ppi density. Its default Retina scaling mode is to run "@2x" assets without any additional scaling, in which case you get the workspace equivalent of 1440x900 but with much sharper graphics. But if you go into your Display Settings, you'll notice that you can also choose to simulate other "Scaled" resolutions, such as 1920x1200, 1680x1050, and some others. Those can be useful if you want to have a larger effective workspace than "1440x900 equivalent", but you'll likely also notice that the graphics aren't quite as sharp -- because in those modes, the native art isn't being mapped pixel perfect onto the display anymore. macOS is instead scaling the artwork to a smaller size (in the examples I gave) to give you more desktop space. If you get a 25" QHD display, you might find that you'll have to run one of those "scaled" resolutions to make things a usable size.
Making matters worse is that while macOS supports running multiple scale factors simultaneously, it can only OPTIMIZE for one at any given time. So if you keep your Retina display in its default mode of 200% scaling and use another display that has any other scale factor, including regular 100%, then your Display Settings area will have an option asking you whether you want to optimize for your built-in display or the external display. That option determines which scale factor macOS renders for internally, and the other display gets GPU rendering instead. So here again, whichever display you're NOT optimizing for will have a lower quality image than it would otherwise, even if you're using an "even" scale factor like 100% or 200%.
The ideal scenario is to ensure that a) you always run an "even" multiple like 100% or 200%, and then ALSO b) make sure that all active displays can use that single scale factor, by making sure they all have similar pixel densities. A 24" 4K display will get you there. A 25" QHD display probably won't look very good even on its own because of the scaling, but it's virtually guaranteed not to look as good as it could if you also try to use it simultaneously with your Retina display -- unless you choose to optimize for the 25" QHD display, in which case your Retina display will take a quality hit.
jphughan
9 Legend
•
14K Posts
3
May 12th, 2020 18:00
@SnapGeeko I find it useful to check specifications in the User Guides / Owner's Manuals when the product pages don't provide any clear indication of differences. But fyi, the U Series (UltraSharp) is Dell's top-end business display category (with the UP UltraSharp PremierColor being sort of a specialized line), and the P Series (Professional) is the next step down. And that shows, because in this case the "2020 model year" U Series has some advantages over the "2021 model year" P Series display:
U2520D:
P2421
I got all that from spending about 5 minutes looking at the manuals of each display.
Fyi though if you want QHD resolution I'd really recommend stepping up to 27". If you look at the dimensions of each display, you'll find that they're not very much larger, but the extra space brings the pixel density down just enough that most people can use QHD without enabling any display scaling, which some Windows applications still don't handle very well. But QHD on a 24-25" display would be tough to use without scaling. Or if you want a really sharp 24" display, then go all the way to 4K. I honestly don't understand why 24-25" QHD displays exist, but I guess I also don't understand why the similarly positioned 27" 4K displays exist either, except I guess to satisfy the market craze for 4K. But a 27" 4K display's pixel density is high enough that you have to enable scaling with all of the potential issues that creates, but not high enough to create that really sharp "Retina display" press quality text experience -- so it's sort of the worst of both worlds. A 25" QHD seems like it would be in a similar position.
SnapGeeko
8 Posts
0
May 12th, 2020 20:00
@jphughan Thanks again for the detailed response.
Curious, when you mentioned "display would be tough to use without scaling". What does it really mean? Isn't having less size and more resolution a good thing for sharp view? For ex: Macbook pro is 15'' and > QHD resolution and text and everything looks so sharp.
jphughan
9 Legend
•
14K Posts
1
May 12th, 2020 21:00
@SnapGeeko I forgot you were on a Mac. What I said applies more to Windows since macOS handles scaling a bit better overall, but even there it's generally better to use even multiples of scaling like 1x, 2x, or 3x, rather than 1.25x, 1.5x, and so on -- which is likely what you'd use on a 25" QHD display. Bear with me here. macOS going back quite a long while was designed with a "reference" display standard of 100 pixels per inch. Windows used 96 ppi. That meant that if you were using a display with the reference pixel density of that OS, then something that was meant to appear 1" wide in the real world -- like the ruler in Microsoft Word, for example -- would actually be 1" wide on that display. And if you look at lots of Apple's pre-Retina displays, like the 23" 1920x1200 Cinema display and the 30" 2560x1600 display -- you will find that they had 100 ppi densities. (They went a bit higher on laptops because laptops are designed for a shorter viewing distance.)
Then Retina displays came along, where Apple basically doubled the "reference" pixel density in both axes. So a display that used to be 1440x900 would become 2880x1800, for example. And to help developers scale their artwork to display properly on those displays, macOS allowed applications to bundle so-called "@2x" versions of their art assets, so that if it was running on a Retina display, it would load those assets rather than just trying to scale up regular art assets. And if you were using a display that has a pixel density of about 200 ppi, then the on-screen content would remain a sensible physical size on the display, AND that "@2x" art would be pixel perfect on your Retina display because it would have been designed with that specific 200 ppi density in mind. A 24" 4K display has a pixel density of 183 ppi, for example -- because it's very close to taking that older 23" 1920x1200 display I mentioned, with its 100 ppi density, and just doubling the pixel density on both axes while maintaining almost the same size. Technically things would be a tiny bit larger than intended if you used the "@2x" art with its default scaling, but it's close enough, and slightly larger usually isn't a problem.
But a 25" QHD display is 117 ppi. That's not a whole lot higher than "reference", but it's probably enough of a step up from reference that if you didn't enable scaling, things would be just a bit too small. But if you DO enable scaling, you wouldn't be able to use 200% scaling in order to use the sharper "@2x" assets, because 200% scaling would make things way too large and completely eliminating the larger workspace benefit of higher resolution. You'll instead probably have to choose a mode that would be something like 125% scaling. But when you do THAT, then none of the art assets available to macOS will be pixel perfect on your display bercause none of them will have been designed with that scale factor in mind. Instead, macOS will use the regular art assets and then use GPU scaling to blow them up a bit. That won't look as good as having a display with a pixel density that allows art assets to be used without any post-render scaling.
You can sort of see the impact of this on the Retina display built into your Mac. The last 15" MBP had a display resolution of 2880x1800, with a 220 ppi density. Its default Retina scaling mode is to run "@2x" assets without any additional scaling, in which case you get the workspace equivalent of 1440x900 but with much sharper graphics. But if you go into your Display Settings, you'll notice that you can also choose to simulate other "Scaled" resolutions, such as 1920x1200, 1680x1050, and some others. Those can be useful if you want to have a larger effective workspace than "1440x900 equivalent", but you'll likely also notice that the graphics aren't quite as sharp -- because in those modes, the native art isn't being mapped pixel perfect onto the display anymore. macOS is instead scaling the artwork to a smaller size (in the examples I gave) to give you more desktop space. If you get a 25" QHD display, you might find that you'll have to run one of those "scaled" resolutions to make things a usable size.
Making matters worse is that while macOS supports running multiple scale factors simultaneously, it can only OPTIMIZE for one at any given time. So if you keep your Retina display in its default mode of 200% scaling and use another display that has any other scale factor, including regular 100%, then your Display Settings area will have an option asking you whether you want to optimize for your built-in display or the external display. That option determines which scale factor macOS renders for internally, and the other display gets GPU rendering instead. So here again, whichever display you're NOT optimizing for will have a lower quality image than it would otherwise, even if you're using an "even" scale factor like 100% or 200%.
The ideal scenario is to ensure that a) you always run an "even" multiple like 100% or 200%, and then ALSO b) make sure that all active displays can use that single scale factor, by making sure they all have similar pixel densities. A 24" 4K display will get you there. A 25" QHD display probably won't look very good even on its own because of the scaling, but it's virtually guaranteed not to look as good as it could if you also try to use it simultaneously with your Retina display -- unless you choose to optimize for the 25" QHD display, in which case your Retina display will take a quality hit.