I've been to Belize (AC only each time) and Cozumel each twice. They each have good diving and other attractions. They are very different topside. Make sure you research carefully to know what you want. I'm not sure which I prefer. Coz probably has more colors and more fish. Belize has more large fish: nurse sharks, groupers, eagle rays. We probably saw an eagle ray on every other dive in Belize. I've never seen one in Coz. Coz has about 70,000 residents, Belize AC only about 5,000, but even with that Belize does have enough nice restaurants to keep one happy for a week. The best diving as has already been posted is at the atolls, but then you are either on a livaboard or one of the few atoll resorts with not much else but diving going on. Maybe that's what you want. I'd glady to go back to Coz or Belize, plan to in fact.