Vista. Reasons why:
* Better security
* Much better networking
* Better explorer interface (at least i like it)
* Voice recognition is almost sci-fi level now (i'm seriously so amazed by it, but hear so little of it from others).
* FASTER. Yes, it's actually FASTER than XP. And i only have a Dell laptop with 2Ghz PM CPU, 2gig ram, and 6800Go Ultra.
* Aero and the GUI (including alt-tabbing, ctr-tabbing, etc... too bad it didn't have cube/multiple desktop functions like linux glx)
Reasons why XP is better atm:
* drivers (my Dell 1350 Wireless isn't working)
* software support of course.
But i'm migrating to Vista asap. Still on the fence about x64 or 32bit for my HTPC though... 32bit because most XP drivers actually work fine in Vista, but x64 for the possibility of better 64 bit performance when more software comes out for it.