I'm gonna say it now, shooters suck when it comes to AI in the campaign or "story" modes, they really do, every single one I've played. For me it's Halo all the way, I didn't like Gears that much, it's good but I hate multiplayer. I love Halo's multiplayer, it's fun and it really is different, most people bash it for being the same but the only thing that's the same is that it's a FPS so naturally it would feel the same. However, I think it plays totally different than any other FPS out there, it has more of a run and gun style which happens to be my style, there is constant action. It plays a lot faster and the shield system puts a little more emphasis on teamwork, though individual skill is also a big part of it, all the little tricks that help out your gameplay is great too. It can also be very strategic though, especially when played competitively and once you get to know the weapon and powerup times as well as the spawn system. Call of Duty 4 is very good as well, but Halo is more my style, not to mention I'm better at it than the other games.
Halo > CoD > Gears