I just recently traded in Halo 3, it's of my opinion that it's really the same thing as halo 2 with a different story. I say rent it before you buy it as it's the sensible thing to do with any game. Co-op gameplay is fun as hell, but only because you have friends with ya. Otherwise I didn't bother with any other aspect of it.
I prefer CoD4 over Halo 3 because it's just a cleaner, smoother, prettier game. The game play is essentially the same with some layout changes and alternate customization choices. Rent 'em both, give them a try and see which you like better. GTA 4 is okay, R6V2 isn't AS good as it used to be, but CoD4 ruined a lot of people.