I like COD4, the guns, the prestige and rank, etc.
But Halo beats it in the fun side of things. If I want to have a good time with my buddies, I prefer Halo. If I just want to kick some a$$ then I'll do COD4, which is just a frag fest.
Each got strengths and weaknesses. I just want to get all my achivements on Halo and get pretty good at it, then get bored and then go back to COD4.