Both. Can't really choose one or the other . Halo 3 offers more crazy shooty action , whilst more call of duty 4 offers more competitive edgy gameplay.
Though they both aren't worth buying if you lack xbox live. Campaign on Halo 3 and Call of Duty 4 are both kinda bad.
Call of Duty 4 hands down. I'm not a big fan of Halo series like I use to be. Plus I get a lot more enjoyment and entertainment out of stuff I can relate to, Call of Duty series (Call of Duty 3) for example - sense it's ww2 and I love history.
Anyway, the gametypes and gameplay and style are preferred in favor of CoD4.
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Everybody call me A-Will, yo sup!
I prefer Halo 3 over Call of Duty 4. Only reason is because I find COD4 to get boring after running and gunning as many enemies as you can.
But don't get me wrong COD4 is a good game, the single player story is very in depth, and the online Multiplayer matches are very intense yet exciting.