I reckon the Dell laptop you're referring to is the old XPS 'Developer Edition'. I used one at work and it was ok, didn't have any driver issues from memory, but they no longer sell them to the public with Ubuntu. It was the same as a regular XPS but with Ubuntu 14.04 pre-installed.
I don't believe they offer Ubuntu as an option with the current generation of XPS's, it dropped off the list of supported OS's. It's a shame really because it was nice to have a premium laptop, with Linux, and with on-site manufacturer support.
What I recall was someone on HN saying the 11 inch Ubuntu model wasn't actually that Ubuntu compatible and to buy the 13 inch instead. But I can't remember was the specific HW was.
I don't believe they offer Ubuntu as an option with the current generation of XPS's, it dropped off the list of supported OS's. It's a shame really because it was nice to have a premium laptop, with Linux, and with on-site manufacturer support.