Different strokes for different folks. It depends on what you are trying to achieve, what your budget is and a heap of other stuff. It's like asking if blondes or brunettes are better!
XR8 is more powerful, around 20kw, that matters to some people but not to others. XR8 might cost more for servicing (2 more of a lot of things, but then 6cyl has turbo so it might even out in the end). XR8 will probably chew through more juice, especially around town and when you are hammering it but it is reportedly better on fuel than the XR6T on the highway. It's an invidual thing. Read
THIS THREAD XR6T upgrade . . . a tough decision
for some other views on the subject.