The Dells are far and away better machines. I have a five year old XPS13 and a year old XPS17. They are great. My only caveat is that I only buy them with Intel/AMD video cards.
The XPS13 is far from "great"; it overheats, the battery life is poor, and the speakers are straight out of the 90s. I have one and I regret buying it.
My dell xps was poorly designed and clearly the focus was on hitting spec numbers than actual usability. The touchpad stopped registering input every few seconds momentarily, enough for a lot of people not to notice. I assumed minor software or hardware fix, they sent 2 engineers to my home who proceeded to replace different parts of the laptop to no success. OS did not fix it. Apparently just poorly selected hardware from vendor at design phase. The overheating CPU was a similar story. Going for thinness over function as its easier to market how it looks.