My wife and I have about 50 dives in the Cancun area. The reef structures are not as elaborate as they are in Cozumel - swim throughs are not as long, for example - but the animal life is essentially the same. We've encounted schools of 20-30 Baracuda, Stingrays up to 5' across the wings, Spotted Eagle Rays, Eels of all sorts and sizes, Nurse, Black Tip, Gray Reef and Bull Sharks, Torpedo rays, Porcupine Fish, Blowfish, Lionfish (which we kill on sight), French Angels, Queen Angels... we even saw two 8' Marlins on a safety stop during our last trip.
We love Cozumel, and plan to be there for 10 days in December. But if Cancun fits your travel plans better, you won't be disappointed. Cancun diving is under-rated, perhaps because Cozumel is so close and so well known.
Get in touch with Alvaro at AlwaysDiving. He'll make sure you have a great time.
And spend one evening at Coco Bongo while you're there. You won't regret that either.