Hmm. I'm debating whether to get this for PS3 or 360. PS3 has Kratos, but no online at the moment. 360, no Kratos, and I have to pay to play online.
I would go for the system where you know most of your friends are playing the game on. Because to be honest that's who you are going to be playing with.
But if you want the cheaper alternative go for the PS3 one. Free online and a extra character.