Dell XPS 15

There's no way to answer that short of auditioning them yourself.  What looks better to one person won't look so to another.

There are many differences between the upcoming 2020 version and the prior generations - the promise of better cooling, for one.  There are also downsides (the new ones are all USB-C, with no "A" ports, for one).  

There are upsides and downsides to 4K screens as well. You do get better resolution -- but you pay for it in battery runtime.

The other big issue is - can you wait?  The lead times on the new models are still quite long, so you're in for a bit of a wait. 

@Duider  If you're concerned about display quality, I would recommend waiting for reviews from sites that are known to go in-depth on the display, such as Notebookcheck.  They've reviewed past XPS 15 models, often multiple configurations to cover different displays, different CPUs, etc.  You can take a look at some of their reviews of past XPS 15 models to get a sense of the information they provide.

But obviously from a pure resolution standpoint, FHD will be lower resolution.  How the new XPS 15 9500's FHD display will compare to your model's UHD display in terms of color gamut, color accuracy, max brightness, brightness uniformity, contrast, etc. is all up in the air.  And fyi you'll have to figure out which model you have.  Based on the year it's probably the 9550 or 9560, but you'd want to confirm if you want to start comparing reviews.

I will say though that I had an XPS 15 9530 with the "not quite 4K" 3200x1800 display, and when I just got a new laptop I switched to FHD.  When I got the 9530 I chose the higher-end display mostly for futureproofing, but years later multiple Windows applications I use still don't work quite right when display scaling is used, as is necessary with that level of pixel density (but not with 15" FHD, at least not for me).  On top of that, the 4K displays use a glossy coating, while I prefer matte/anti-glare to minimize distracting reflections.  They also include a touchscreen, which seems utterly pointless to me on a non-2-in-1 system since it's tiresome to hold your arm in midair to operate a touchscreen on a traditional laptop.  And perhaps most importantly, the 4K displays take a major toll on battery life, normally about 1.5-2.5 hours, depending on the exact system you're talking about and of course how you use the system, on top of adding a bit more thickness and weight.  So when I was ordering my new system, I realized that at least for my purposes, the upside of the 4K display was that for applications that scale properly, things look nice and sharp.  But for applications that do NOT scale properly, they will sometimes look worse than I would if I had just had the FHD display -- and meanwhile I'm dealing with all of the other drawbacks of the 4K display -- and paying more for the privilege!

If Windows was today where macOS has been for years in terms of handling high pixel density displays, then I might rethink my choice -- maybe.  But although Windows and third-party applications have been getting better over the years, there are still many cases of applications not displaying as expected when scaling is enabled.  macOS was able to deliver this feature basically flawlessly because Apple has full control over the OS and the hardware, and they were basically able to ensure that they only had two "classes" of pixel density across their display offerings, namely non-Retina (100-120 ppi depending on desktop vs. laptop display) and Retina (200-240 ppi).  That in turn made application development much easier, because Apple basically built a framework into their OS that allowed developers to include "standard" and "@2x" versions of their artwork, and the OS would just use whichever was appropriate (or the nearest match when using a PC display with a Mac, where PC displays can have a whole range of pixel densities).  Windows doesn't have that.  It's dealing with a huge range of pixel densities because there are displays of a given size that might have all sorts of available resolutions -- 27" displays can be had in FHD, QHD, 4K, and 5K resolutions, for example -- which is partly why Windows today still doesn't handle display scaling as well as macOS managed years back.

I'd personally wait for more reviews on the new XPS because you'll get a better idea as previously said above on pro's and con's.