I have both systems, and I would recommend xbox. I have played both of them TREMENDOUS amounts of time. About 95% of games come out on both systems, and they look better on Xbox, with the exception of the only game I can think of, Need For Speed: Hot Pursuit 2 ... it was much better on PS2.
Along with better graphics, on Xbox you dont need a memory card, and the load times are noticeably better. The DVD playback on the Xbox is also much better than the ps2.
Most of the "must have" PS2 games end up coming out on XBox in the long run (like GTA and MGS2), with the exception of a few. IMO, the exclusives on XBox are actually better. I am a huge EA fan though and EVERY SINGLE EA game, cept NFS:HP2, is better on Xbox as far as graphics and load times, and the memory manager on the XBox is great because as you probably know, EA games take up ALOT of space on a PS2 memory card.
My ps2 mainly collects dust now a days, with the exception of some ratchet and clank 2 because I haven't finished it. Besides that game and Jak 2, I hadn't really touched my PS2 in a while.
So in my opinion, I would have to go with the XBox. And if you dont like the xbox controllers, which I know alot of people dont, you can always get the PS2->Xbox control adapter that lets you play with your ps2 controllers on the xbox, for about $20 or $15.