I have a SurfaceRT with Type Cover, I have a Dell XPS 12, I've played with a Surface Pro. Here's my thought:
XPS 12 is the way to go unless you really, really want a stylus...which has its uses.
The keyboard on the XPS12 is far superior to the type cover. It has back illuminated keys, its larger sized, it disappears when you go into full tablet mode. The Surface type cover, while nice, can't compare to one with enough depth to give as satisfying a keystroke. I don't like folding the keyboard under (exposed) when you want to use the Surface in "pure tablet" mode --though I think this is where the pen would be at its best advantage and to me detaching the keyboard is nice but not as practical if you're not walking with a bag to stow it in. I also find the clamshell design of the XPS makes it much more comfortable and solid when used as a LAP-top. The touch and type covers seems awkward to me when resting on my lap instead of on a table.
The screen on the XPS is much larger than the Surface which to my slightly aging eyes is a good thing, or at least makes the side by side apps more enjoyable. I've no issue either way around the quality of the display. Both are bright enough, both have good viewing angles. The XPS has a physical Win Button under the display while the surface uses a synaptic one and I don't have a strong feeling in either direction around that.
One interesting bit though around the viewing angle: with the kickstand, the Surface basically offers two angles, flat on the table and kickstand angle while the XPS can be tilted back slightly farther than surface (it cannot lay flat with keyboard exposed). At its maximum angle the XPS is delicately balanced -a touch to the screen is going to cause the XPS to rock backwards where the Surface will hold firm thanks to the kickstand.
The XPS has two USB both located on the right hand side --more comfortable for a physical mouse, while the Surface Pro has one on the left hand side. Both sport Bluetooth, both sport at least decent Wifi connectivity.
With the XPS, there are slightly more options around SSD and RAM -mine has a full 256GB SSD and 8GB of RAM though I stopped at the i5 and not the i7. The Surface has the removable card, but I feel less confidence in putting data on the card than the internal SSD. I've not looked at expected lifetime of the internal SSD v. the SD card...anyone have insight on that?
Hope this helps.