I might be the odd one out. I have a macbook pro retina running ubuntu 14 using parallels. Could not happier.

Assuming budget isn't an issue, I find many linux laptops (including the 13inch Dell) to have bad UX. Relatively short battery life, low res screen, and may i say, terrible trackpad.

Apple trackpads are significantly better than all competitors.

On top of that, a lot of nice to have apps don't run on linux. Things like screenhero. And apps that do have a linux build often don't work as well. (Think skype, hipchat)

Running VM on macbooks is the best of both worlds, you get linux for development and OSX for everything else.