I'd give the nod to Cozumel over Key West. The Coz experience goes beyond the diving as the town is a cool place to hang out, and there are some great places to eat.
However Key Largo is a great vacation dive spot. I'm not sure I'd say the diving in Key Largo is a lot worst than Coz, but it's different. One thing Key Largo has going for it other than it's in the US is that there are some great wrecks to be done.
What Key Largo is missing is the atmosphere of the City of Coz, or Key West. I've not dove off of Key West, but I hear it's no where near as good as Largo.
Coz has MUCH more dramatic reefs vs. the Keys. But the Keys have some very amazing reef diving especially when VIS is in the 100foot range. Some things I enjoy about Key Largo diving is an abundance of Jewfish, and Nurse sharks not generally seen as much at Coz. Key Largo also has some REALLY nice Brain Coral, and I did not see anything in Coz as large. Coz OTOH has more turtles, or at least I saw a lot more there. They also have a lot of spotted eagle ray, or again, that was my experience.
I don't think you can go wrong with either Coz, or Key Largo. For a diving Trip however I would not think Key West would be a 1st choice.