Yup an X6 would require a donor B4, as would a Centro or Vega so that would need to come into the equasion when it comes to making the decision. Quite rightly though the X6 is still an extremely competitive car & backup is second to non
Personally I'd go with the Durango, I run a 410 already so I'm familiar with the brand & know plenty of folk who run them so no problems when it comes to setup & should I have a problem.
Lots of good platforms out there, plenty of choice! Best bet would be to see what the guys at your local clubs are running and get talking to them, as well as giving their insight into the cars, after you've chosen your route they will also help you to no end once you're up & running