I'd say PS3, since multiplayer isn't really something you want if you do want to dabble a bit it's free on the PS3.
Also the PS3 you can watch movies directly off a thumb drive or external hard disc so it makes an excellent media centre.
Most games are released on both platforms except for the few.
Its a real shame you couldn't get a VLC type app for the PS3, would make it a really good media centre