I personally prefer Vista now, but only because it feels new. XP just seems so outdated to me. I know it's faster, but I just like the look of Vista more than XP, so when I can run it... that's what I choose. Vista still doesn't work well in a VM, but hopefully that'll change as we get more powerful machines, or the Parallels/VMWare/VirtualBox teams really kick the coding up a notch.
Either way, Vista gets my vote in Bootcamp.