If you have a smallish hard drive, you might want to consider getting 2000.
I saw a brand new Gateway laptop with XP Professional on it today, and the install size was ~ 1.2 gigs! I realize the install is customizable, but I've heard a lot of similar stories.
Not only that, but XP should be penalized because in its original format, it's pretty/animated/hogging resources/flaming/etc.