The reason to own a XBOX: Halo, Fable, NO memory card needed (that is a HUGE plus) The built in hard drive is like owning 30 PS2 memory cards. Better graphics, much faster load times, Riddick, Ninja Gaiden, Rallisport 2, Project Gotham racing 2 (along with Burnout 3) is the best racing game made so far on any system, Crimson Skies, HALO 2 is coming!!
Reason to own a PS2: More games available exclusive to the system (but the graphics are better on XBOX), Some people like the PS2 controller better, Suikoden III, Devil May Cry, Onimusha 2 and 3, More RPG's available, It's the number one system in Japan so Japanese game developers focus heavily on making games for it.
I have every system made since Colecovision and will buy every one that ever comes out if I can. I personally like the XBOX better because it is just the best quality of graphics and the most powerful system available right now. In my opinion, the PS2 is old technology compared to XBOX. Play Riddick and you will see what I mean...The PS2 is just not up to par with the capabilities of the XBOX.....Granted the PS2 has some great games though and what's a powerful system without games? The Xbox has some incredible games as well and they completely rock as far as being up to date with technology's abilities now.
I would say if you are into RPG's and Japanese style games, get a PS2. If you like the best game graphics, first person shooters or any other games besides RPG's, definitely get a XBOX.
You can't go wrong with either system, but if you decide to go PS2, think about waiting for the new smaller PS2 that is coming out in November and there should be a price drop when it comes out....Just my opinion from my experience with both systems.....hope it helps