I've got a couple of Dell 24[something]s which are 1920x1080.
If you are running Win10, you can now set the scaling level independently for each display, meaning that I have the SP3 internal display at 150% scaling, with the externals at 100% scaling, so things are about the same size -that is menu and toolbar items don't end up too big on one display or too small on the internal display, as they used to on Win8.1.
So you can set your preferences per-monitor, and Windows remembers them. That's very good news: the thing that hasn't changed is that you really need to log out, then back in again, before all applications are totally happy.
If I just dock and carry on working, there are some things that have fuzzy text or or other anomalies.
It doesn't worry me much as I typically dock when I'm about to settle for a bit, but it's not the seamless wonder-demo that it could be.
(also I'd note that my wife -we share the dock - never logs out on docking, and the anomalies don't seem to bug her at all, but her eyesight isn't great)
Back to your original question, either resolution will work fine, as I suspect you will choose to log out/in for general reliability.
It comes down to preference or budget - given the option now, I'd always choose the higher res if drivers and budget allowed.