Halo4's MP is amazing, and it gets better every week, as they add more maps, more Forge maps, new amazing modes (if you haven't played Team Throwdown and Action Sack, for example, you're missing out greatly.)
Halo 4's only issues are the skill gap is great. New players don't have a lot of fun, because they just get wrecked. And when they launched, it was somewhat slimmed down in terms of playlists, and some of the best stuff didn't come online until their weekly/biweekly updates along the way.
I like shooters a lot in general, and I'm a big Halo fan. The package Halo offers to the people that really use it/explore it is just amazing compared to most console shooters. Just in matchmaking right now there's something like 30-40 maps available, and only a handful (6?) are paid downloads. In file share there are literally a hundred maps worth playing. COD is just easier and simpler to play and understand and to occasionally feel like a god in. And seriously, the level-up system of COD is super addicting - just like Farmville and facebook games. Constant reward. Even though they implemented these systems in Halo 4, they're much slower and much less obviously affecting gameplay (which is good, in my opinion, for balance reasons).