Which docking station for Latitude 5490?

Short answer:
If your 5490 does NOT have the Thunderbolt 3 option (and/or you don't expect to need external display setups beyond dual FHD displays), get a WD19 with a 130W AC adapter.  If your system DOES have Thunderbolt 3 and you might want to run higher-end display setups, such as dual QHD or dual 4K later, then get the WD19TB.  Unfortunately if you don't have Thunderbolt but want the option to expand to higher-end display setups, then you're a bit stuck unless the D6000 would be an acceptable option for you despite its drawbacks.  See below.

Long answer:
The WD15 and TB16 have been replaced by the WD19 and WD19TB, respectively, so I don't see a good reason to look at them anymore unless maybe you can get a good deal on the used market.  The D3100 is ancient and has been replaced by the D6000.  The problem with the Dxxxx series docks is that they use DisplayLink chips rather than tapping directly into the GPU, which can introduce a lot of drawbacks.  I wrote in detail about those in this thread, specifically the post marked as the answer -- and even though the thread asked about gaming performance, the drawbacks I mentioned are not limited to gaming.  The only situation where I'd suggest getting the D6000 in your case would be if you needed an external display setup beyond dual FHD, did NOT have Thunderbolt 3 on your system, AND could accept the drawbacks I mentioned above.

As for power, the WD19 when powered by a 130W AC adapter can supply up to 90W to the attached system, and if you check the AC adapter that came with your 5490, I suspect you will find that it's either 65W or 90W.  Either way, a WD19 with a 130W adapter will be sufficient to power the 5490.  The WD19 with a 180W adapter by comparison can supply up to 130W to the attached system, which is required by systems such as the XPS 15 models.  The WD19TB only comes with a 180W adapter at the moment, it seems.  You won't need that extra capacity, but it won't damage anything to have it.  The D6000 however can only supply 60W to an attached system.  That will probably be fine if your system came with a 65W power adapter (the battery will probably just charge a bit more slowly), but if your system came with a 90W power adapter, then only receiving 60W while docked could cause it to throttle back its performance in order to try to operate within the reduced power budget.  You could work around that by connecting the AC adapter to the system in addition to the dock, but of course that defeats some of the convenience of the dock.

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@dch82 wrote:

Thanks for your reply, it's much appreciated!

It's in line with what I had understood, so that's good news! I take your point on the Dxxxx series docks so I don't think I will go down that path. The Latitude 5490 doesn't seem to exist with Thunderbolt, so that would leave me with 2 options: either the WD15 or the WD19.

The only issue I have with the WD19 is that it's really expensive (330€ on Dell's website) and since it's recent, there isn't really any used market (yet).

On the other side, the WD15 "only" has mini DP, VGA and HDMI, so I would need adapters as well...

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@dch82  happy to help!  Sorry, I thought the 5490 had Thunderbolt 3 available as an option, but maybe that's only the 7490.  Anyhow, in that case if you're ruling out DisplayLink, then yes the WD15 and WD19 are your best bets.  The regular WD19 doesn't get you much over the WD15 when used with the 5490.  The main new feature is support for DisplayPort 1.4/HBR3, whereas the WD15 only supports DisplayPort 1.2/HBR2.  HBR3 allows more video bandwidth from the system, which in turn allows the WD19 to support display setups that require more bandwidth (dual QHD rather than dual FHD, for example), but the 5490 only supports HBR2, so that benefit wouldn't apply with that system.  Therefore if the extra cost of a WD19 isn't worth the futureproofing for other systems, you should be fine with a WD15.  All you'd need is a Mini-DisplayPort to DisplayPort cable/adapter and an HDMI to DVI cable/adapter.  Those are very inexpensive, especially (I expect) in comparison to a new WD19 vs. a used WD15.