I've used both, for extended periods. XP since 01 and Vista on and off for the last year (about 8 months of it...), and I do have to say both are very similar. Vista takes more resources, but does have some nice browsing/looks. Support is better than earlier this year, but there are still issues. If you like to game and know you're getting the best performance, XP is the way to go for now.
I will go back to Vista (probably on a secondary boot drive) to play around with it again, as I do like how it works, I don't like what it does though. DX10 is not worth Vista, but then again I don't have a DX10 card as of yet. Really, you gotta try it for yourself and see if it's what you want or not. Just be prepared for a let-down or two, or a limitation here and there. As I said it's a lot better than it was IMO, and it can only improve...XP was the same way when it was released, I just don't think Vista will have the staying power of XP in the long run and that MS will in-fact replace it sooner than later, which would be a good move imo.
But also noted, my 3d gaming performance wasn't that much worse in Vista recently, and in some cases was almost dead-even...and as long as I didn't mess with Alchemy too much, most of my games were fine.
You'll be fine if you do your research and expect or are willing to deal with issues that may be un-avoidable.
