Hi there, and welcome.
Just been looking around on the site for some interesting threads and came accors something in that line a while ago,
Go have a look at :

http://www.xt660.com/showthread.php?t=9958

There is some advise and talk about this issue in the general section, as well in others i think...look under Preparing XTR for long trips, and Riding techniques as well.

But i can vote for the XTR being pillion friendly compared to oither bikes. The Transalp seat might be more suited for pillion, but with a little bit of determination your misses will get used to the XTR seat at the back....i love the rubbing part after a long day in the seat...
GB