there both good systems(sadly your forgetting the timless cube`)
depends on what you really like. ps2 has awsome platform adventure games, the GTA series, THUG, socom, ace combat, Final fantasy and a strong online presence.
the X-box has strong online lineup, and has the best shooters, good RPG's(non FF battle type anyway), a solid sports line up with slightly higher graphical dispay...anyway I could go on about all the good stuff for both consoles but I wont.
It really comes down to preferences. Do you like the sony controller?, do you mind all the money to go online? do you enjoy the broad content of games? then buy a Ps2 if you said yes.
Do you enjoy a out of the box online, the controller, and a over all strong presence in all gaming fields except flight sim, and paltformer, get xbox.