i would go with the 360, halo 3 is a pretty strong game with pretty good multiplayer and forge is also amazing if your into custom games with friends. The single player is pretty run of the mill but amazing graphics. The only flaw is the nasty online community of in-mature kids, but theres always a mute button
CoD4 has breathe taking graphics! the single play is great but kind of short, and the multiplayer has a sweet ranking system that goes the more kills and captures you get the better weapons you will recieve, and the community is not driven by kids.