IMHO, XP is simply faster... less optics, less mechanisms going on inside.
Vista has a better design, also many safety options (which most users won't ever see, though, as they won't look there) and so forth.
The major drawback for Vista is that you really need new hardware (best not older than 6 months) to get it running well... at least for the 64bit versions.
I had a 1 year old mainboard, and sadly no Vista drivers
well, so no ethernet or what? Oo
Though, if I'd had bought Vista (got it free from the University) I might have tried out going to Linux, since it's free.