I only buy dell if I expect linux to work at all any more, even then it's hit and miss (as I stare at my xps15 in an arch linux install cd after a week).  Thinkpads tend to have their issues it seems too, lenovo doesn't so much seem to care about linux, and is left to hobbiests to figure out, which only goes so far.  Dell at least has folks that work on the kernel team to get some real input I've found.  Using the ones they sell with linux obviously goes a long way toward support, like the xps's, at least with ubuntu and such they tend to support.