From what one of the guys I used to work with told me(he was beta testing it), to fully utilize Vista, you need four gigs of RAM. That alone led me to believe XP was the better choice...
I will be testing Vista next month with our products. I've heard some good things about it and some horrible things about it. I will try to remember to give a report when we are finished. The upside is that I get a new computer so that I can test Vista.