i have had all 3, and currently just have xbox and ps2, but my ps2 collects dust. i would say if the target audience is over 13 years old, either ps2 or xbox will be fine. i personally play xbox for the following:
-xbox has better graphics than ps2, and alot of games are developed for both systems and they always come out better on xbox
-xbox loads alot faster than ps2
-xbox does not need a memory card, because of the hard drive
-xbox does not require an additional purchase to get a network adapter, its built in
-ps2 has more/better exclusive titles IMO(with the exclusion of Halo 2 and some others), but i also have xbox so thats not a problem.
but honestly, when I think about it, the majority of the games that I personally play are on both systems, and like I pointed out, they always are the best on xbox.
aside from that, if you want to take your console online, get XBox. I just got xbox live like 2 months ago and I now regret not getting it sooner. it is the most fun I have had with a console in years. and if you are going to go with the modding scene, xbox is way better because you can virtually turn it into a media center.
but i just honestly haven't even wanted to play ps2 in months, even with GTA:SA out. I played it at a friends, but then i went back to burnout 3 and Mortal Kombat Deception on xbox live. xbox live will have you playing games you never really thought you would play.
my vote is for the xbox.