Now before you bash me, I was one of the people that got Halo 3 at 12:00 at launch.

With that out of the way. Halo 3 was a really big disappointment. The singleplayer felt like a chore and wasn't that fun. Multiplayer gets boring, with the only exception being Infection. The weapons are unbalanced, someone with a sword and a needler can win the whole game. It helps a bit that they made the sword like Campaign's, though.

Now, CoD4 singleplayer was a blast. I loved all the missions (the Mile High Club being my favorite). Multiplayer never gets boring. Also, giving you weapons when you rank up is awesome.