There is more to the issue than external monitor compatibility.  Unless you plan to use the external monitor exclusively (and either an FHD or UHD internal screen won't really affect that), there are reasons for choosing one or the other: UHD resolution comes at a significant cost in battery runtime, and unlike a discrete GPU, it's always on, so you cannot economize on battery runtime.  FHD is far better for battery runtime. The other downside is that UHD screens don't always play well with some applications -- perhaps not an issue if you plan to use the external monitor exclusively.  
Hi... I am going to purchase a new xps 15, however I have heard of issues when connecting a 4k laptop to external monitors where the resolution plays up. I am happy to get a 1920 screen on the laptop if that is more reliable than getting the 4k or 3.5k screen. Also the monitors are thunderbolt type connected. Any suggestions on the screen type?  
What graphics for external monitor compatibility (new XPS 15)