In my experience, XP is faster. I've used Win2k, and while it's a much more mature OS, XP is by far the faster of the two, on all the machines that have done the 2k to XP changeover.
I'd have to say XP is faster provided you turn off the Luna interface. It has a faster boot time, and a bit better performance than 2k. With the Luna interface on you'll suffer a bit of a performance hit but unless your running a p2 266 and 128 megs of ram it shouldn't really be that noticable of a difference. Considering XP is just an enhanced 2K core they are fairly close (minus boot time), and either are fairly fast provided you have the RAM.
I now installed win xp and I must say its alot faster then win 2k it startsup within 45 secs But shutdown is slow , but that was in win 2k the same. So im keeping win xp