If you are into 80's and gaming violence then GTA: Vice City PS2 is a reason worth buying PS2. I mean in what other video game can you jack a car, run from the cops, and then stop to pummel the hell out of innocent pedestrians -- or run over them DR 2000 style -- that get in the way of you running away from the cops (while cranking up Judas Priest: You Got Another Thing Comin')?
Everquest Online Adventures (beta 2 right now) is looking -- and playing -- pretty sweet for PS2. It will come with a monthly subscrip fee, though, and the word on XBox live so far is all their games will be across M$ servers so no 3rd party fees mo yet, so that's something to keep in mind.
Also, XBOX controller is a bit, er, awkward and their library of good games is a lot smaller than PS2. However, the hard drive as mentioned is nice and you can burn all kinds of CDs and listen to those while playing many games.
Gamecube lost Rare to M$ which will tip the scales more in XBox favor when Rare starts cranking out some titles for M$ because they are an awesome developer.