call of duty 4
I got Halo 3 a month before Call of Duty 4, but I quit playing right after I got Call of Duty 4.
Every aspect of COD4 is better than halo including the single player (besides the fact that its shorter than Halo3 - quality > quantity)
There aren't any douche bags with snipers who will headshot you while moving from the other side of the map. In Call of Duty 4, the snipers are harder to use since there is breathing, etc.
Also, the fights are much shorter which is better since there isn't such thing really as running away, getting your health up, and running back
The weapon attachments / perks / camos are some of the coolest things since you can really customize your weapon to your liking / use
All in all on a scale between 1-10, I'd give COD4 a 9.5 and Halo 3 a 7
I never get sick of multiplayer COD4
If you are with lots of friends, halo3 is he game of choice since there is 4-player per XBOX system link. Call of Duty 4 - you have to have a separate XBOX / TV for each player if you do system link (or you can play 4 people on 1 xbox).
If all my friends could have an xbox / tv though, I'd much rather play call of duty with them