It is all about the games you like. Games like Call of Duty, Grand Theft Auto and stuff are for both consoles. Final Fantasy has been on the xbox. The real thing xbox has is Halo and Gears of War which are outstanding games.
Playstation has a lot of good games as well so it really comes down to which you like more.
I have heard playstation's online is free whereas xbox live is not. I personally have an xbox but that is because I liked the games better. The 2 series I really played for the PS were Resident Evil and Final Fantasy, both of which have made an appearance on the xbox now.