xbox live is the best as far as online console gaming... its unified, you can see all your contacts online and send them invites even if they're in the middle of another game. So instead of having just the people in the chat room of the online service of the exact game you happen to be playing, you have all your online friends all the time etc... so, if you're big on that stuff, xbox
As far as game selection, the ps2 has more, especially rpgs, but right now the xbox has quite a line up of high quality games too.