It is opinionated. Here are some reasons to buy and not buy each:
Call of Duty 4, because it has High-quality story mode packs in a lot of thrilling and unexpected moments, Well-designed multiplayer progression gives you something to shoot for online, Terrific audiovisual presentation.
But its Campaign mode is much shorter than those of the average shooter.
Halo3, because it has a Satisfying storyline that simultaneously provides closure while making you wonder what will happen next, Four-player online co-op is a lot of fun, Forge mode map editor lets you do some extremely crazy and unexpected things, Web integration makes sharing new maps and custom modes easy.
But Rewinding saved films doesn't work quite like you'd expect it to, AI-controlled Marines can't drive to save their lives (or yours), Inevitable flood of prepubescent online players sure to hamper your enjoyment of the online modes.
Hope this Helped!
