Call of Duty 4 is, in my opinion, the better game.
-I'd give Halo 3 a 9.5/10, and I'd give Call of Duty 4 a 9.99/10
-Cod4 is very intense, and it is cool even though it is not really like modern warfare (seems to be set 20-40 years in the future)
-Halo 3: It's not real, but still intense
-Cod4 has a larger online community
-the weapons in Cod4 are much more diverse
-BIG ONE: Cod4 has an EXP system for online that does NOT hurt you if your teammates do bad. You still get your experience points. You must win to get XP in Halo 3, which is bad
-Cod4 is much more challenging
Get both games, but either way, Cod4 is the slight winner.
Happy playing!!