Depends what you are doing. However personally I like XP. If you like the look of vista, wait a month or so. Service Pack three is coming out, (there is a leaked version right now) and it makes XP look just like vista. Maybe not quite as flashy.
What are the specs on your computer? And what are you primarily using it for? I'll assume gaming...
If you have a crazy new computer, with a honking graphics card Windows Vista will let you use Direct X ten. That will boost the newer games that are compatible with it, if you have a DX10 graphics card. Windows XP does not and will not support anything beyond DX nine.
I have a ridiculous setup, and a crazy graphics card and I still use XP just because I like playing my older games. You will loose performance on vista, period. It's better then it was a while ago, but bottom line the same computer will bench a bit slow on vista. This will probably change eventually.
I would stick with XP, for now it can run anything well, and as Poly said, it uses less system resources. If you have a computer with 4gigs+ ram then you will not have to worry about vista sucking resources. Might not even notice a difference with 2gigs. If you are buying a new version from scratch, also keep in mind that upgrading now will save you the money to upgrade in the future.
Also note that at the rate people are switching to vista, XP will be around for a while yet.