I disagree. I think XP is better once you get rid of the kindergarten looking interface. The XP Pro kernel is the same architecture as 2000 (only more up to date) but the interface is a hell of a lot snappier. You also have a smaller footprint and the ability to do user switching which I find valuable.
I will say this though, both are good operating systems and I've heard people complain that they can't run x application in 2000 or they can't run y application in XP. So I will say this, if 2000 runs your app better than XP, go w/ 2K. Same is true for the inverse.