PS2 has alot of favorites carried onto it from the PSX. PS2 has become rather expensive. And by that I mean at first you had to buy a system, mem card, and then eventually a network adapter if you wanted to play online not to mention games. Now it's different with the redesign.
It does have really awesome games, God Of War, FFX, Twisted Metal Black, The GTAs. Some of those greats like the GTAs are on Xbox now. PS2 is a great system no doubt.
XBox always has been ready to play right out of the box (minus a game of course). The only set back would be a cost for XBox Live which for avid players is totally worht it. It has the best graphics of the 3 (though gamecube rivals it with the likes of Resident Evil 4 :-). ) and it's focus seems to be FPSs such at the great Halos, Unreal, and more recently Doom 3. It's a great system for RPGers as well. Fable, Elder Scrolls III, KOTOR I & II, this system is deffinetly awesome.
Both systems are amazing. And I am glad that I have both to play and experience the fun factor delivered by them :-)