Precision 5510 docking stations

Use the TB16 if you want dual displays up to 4K @ 60 Hz.  Use the WD15 if you're ok with dual displays being limited to 1080p, since the WD15 costs less (and will work on systems that only have USB-C but not Thunderbolt 3).  Do not use the D3100 or even the D6000 unless you really need compatibility with other systems that do not have USB Type-C, because those docks use DisplayLink chips for the external displays, which introduce a number of drawbacks compared to using USB-C/Thunderbolt docks where the displays are directly driven by the GPU.  The D6000 also doesn't provide enough power to charge the Precision 5510 (it only provides 60W whereas the 5510 requires 130W), and the D3100 doesn't provide any power at all, so with either one you'd need to connect both the dock and AC adapter to the system separately.  The TB16 and WD15 can each power the 5510 without needing an AC adapter also attached to the system, as long as you get the version that comes with the higher-wattage AC adapter.  For the TB16, that means getting the 240W version (not 180W) and on the WD15 that means getting the 180W version (not 130W).  As you may have surmised, the TB16 dock itself consumes more power than the WD15 -- but as long as you get the right adapter, either of those docks will be able to fully power your system.  Typically the dock price is the same regardless of AC adapter, and the dock themselves are identical; it's just that the AC adapter is physically larger.

The TB16 has had bad reviews because it took a while for drivers and system firmware to stabilize to the point that things worked reliably.  More recently, Dell issued BIOS updates for a few systems that introduced USB connection stability issues with devices attached to the TB16, but the BIOS updates to fix that problem are now out.  I've personally found the TB16 to be rock solid these days, except for that recent hiccup.  Just make sure your BIOS, Thunderbolt software, Thunderbolt firmware, and drivers (ASMedia USB, Realtek Ethernet, Realtek Audio) are all up to date -- and if they're not, update them in that order.