Specifically the XPS13 for the Dell. The 15 is NOT a good option, due partially to it's weight, but mainly due to the poor battery life and the large power supply which means you can't use it on a plane. The XPS 2-in-1 is also OK, but has no standard USB ports (only USB-C) and a very flat keyboard which some people dislike.

I previously worked for Dell and had the XPS15 (technically the Precision M5510 which is the business version of it) and ended up swapping for the Latitude 5480 which I absolutely loved. I now have the Thinkpad X1 Yoga and it's very good, but I'd take the Latitude 5480 over it any day of the week! (The current model is the 5490 I think)