Definately 2000 over XP.
I won't cut & paste again, but go look for my posts in one of the recent "will you move to XP" threads.
Win2000 is a good OS, WinXP took that good OS and screwed around with it a bunch. WinXP is still better than Win98/ME, but it's not as good as Win2000.
Win2000 is the best OS Microsoft has ever produced, and from the looks of the way they are going, it is likely the best OS they will ever produce.