If you want graphics, go PS3. If you want multiplayer that is decent, go 360. I have both and will be getting it for the 360 because XBL outweighs better graphics for this game in my opinion.
Honestly, the graphics thing only applies to PS3-exclusive titles. It's generally accepted that the Xbox 360, although the less powerful console, actually looks and runs most multi-platform games (e.g. Call of Duty series) better. Apparently it's due to the PS3 being more difficult for third party developers to program, while the Xbox 360 is much more accessible.
Oh, and the only advantage the Xbox 360 has over the PS3 for multi-player (in my opinion) is the ability to chat across games via party chat. Other than that, PS3 users get to save $60 a year unless they subscribe to PS+ or whatever it's called.