I'd love to give you an answer, but all the XR6Ts Ive seen havent been so keen to race me Obviously the GTT has the weight advantage, and I'd assume probably better handling. I've heard of a LOT of mechanical problems with the XR6Ts, although apparantly some of this has been fixed in the new version? Might be worth checking out.
Sucks that you need the room for family, otherwise I'd say go the XR6T Ute, as I'm sure they would haul alot of arse Anyway, GTT has oodles more style