If you like short boat rides, crystal clear water, drift diving and cheap prices, Cozumel wins hands down! Diving conditions are not even comparable, at all. Belize is loooooong boat rides, the hugely overrated "Blue Hole", average diving conditions, and also pretty cheap. On a scale of 1-10 I'd give Cozumel about an 8 and Belize anywhere from 2-6, depending on what cay you stay on. Ambergize (sp) is one of the best but the diving is still not in the same league as Cozumel.
You have nicely highlighted some of the advantages of Cozumel.
If you stay on Turneffe or take a liveaboard, boat rides are not an issue in Belize. Your 1-10 ratings make me think you haven't done the best diving in Belize. Have you been to Painted Wall, Silver Caves, Long Caye Wall, Que Brada and Dos Cocos off Long Caye? Or Half Moon Caye Wall and Tarpon Cave off Half Moon Caye? Or the Elbow off Turneffe. These are among the best dives in Belize, and they are a hike from Ambergris or the mainland. In terms of the health and diversity of the reefs, I think they are clearly superior to Cozumel. But
you'll have to pay more to see them,
you won't get that crystal clear water that Cozumel has (although the visibility can be quite good at times), and, with the possible exception of the Elbow,
you won't get the high speed drift. In about 65 dives in Belize, I have seen Hammerheads, reef sharks, dolphins frolicking among the divers on a few occasions, a Saltwater Crocodile (on the surface), ubiquitous schools of Tarpon and jacks, large schools of Permit, Spanish Mackerel and other game fish not normally seen on Caribbean reefs, squadrons of Eagle Rays, and the usual assortment of turtles, octopuses, and other smaller animals. I didn't get that in Cozumel. It is not an apples to apples comparison though. If your budget is limited, you're probably better off going to Cozumel than doing the cheap diving in Belize. If you can afford to pay up, Belize offers superior diving, in my opinion.