The CoD franchise has effectively been a level expansion pack for some time, and very little replay value. The online multiplayer element is (largely) the same from game to game. They release a game pretty much annually.
GTA on the other hand, bases itself around an open world which in itself you'd take several weeks to get bored of. GTA is pretty much a release of multiple stories, with a vast new city each time. They also don't churn out a slightly rehashed version each year.
Saying this as someone who'll buy every installment of both series. But as an occasional gamer CoD I'll be done with in a month, GTA will last me at least 6.