I have to say, the only reason i believe halo has a richer storyline is because of the sheer number of sequels and prequels that have been produced revolving around the halo universe. If Call of Duty was managed by one Developer, and they built the games in the same way halo did, developing that universe and making the relationships we have with the characters of halo and so forth, they would be very close to halo. The campaigns in Call of Duty are far more entertaining, but they just don't have the room to innovate like halo does because of their need to stick to being realistically modern.