Either really.
I'm kind of annoyed by XP because some of my software won't work in it...
EZCD Creator 4.0d, Norton Antivirus 2000, and a few other little utilities like that.
I know there are newer versions that do work, but except for WinXP compatibility, I have no desire or purpose for getting then.
On the other hand, Windows XP works with some games that Win2000 doesn't (this hasn't been an issue for me, 2k works will all my games fine, but I know some people that have had some success).
WinXP has the really nice driver rollback (I don't like the whole system restore thing, just this little bit of it is really handy for someone that plays with Beta drivers alot), and quite fast boot up times. But it has a lot of runtime bloat that I could do without.
I'm running Win2000 Pro SP3, and don't plan to upgrade to WinXP because it doesn't seem to have anything to offer for me personally, so it would be a waste of money (I buy my software, even Windows and Word).
But overall the OSes are pretty much equal. You just have to look at the minor differences and see which appeals to you more.