How deep is deep? They have been to 85 ft and are comfortable in the water. but less than 50 dives. I just want a safe shop that doesn't leave people behind. Do you really prefer Cozumel?
As other have said, the first dive tends to be down in the 70-90 foot range in Cozumel. The dives there are almost all drift dives. Lots of people refer to the diving in Cozumel as "lazy man's diving," since basically you fall off the boat, do just enough swimming to keep the group together as the current pushes you along the reef, and head back to the surface when you're done and the boat that has been following along picks you up. Make sure you carry a safety sausage; even if you were left behind (unlikely, though certainly not impossible anywhere), there are generally enough dive boats around that one would see you and come over and make sure you were ok, and if needed, call your boat on the radio, especially if you had your sausage inflated. The current there varies from very weak to very strong depending on the day and the dive site, but since you aren't generally fighting the current but letting it push you, usually isn't a huge problem.
For Belize, I can only speak for Ambergris Caye, where I dove back in November. The coral there is mostly deeply-cut finger and groove structures. A number them are 40 or 50 feet tall, so people can find their own comfortable depth on the "wall." Most of the sites were 5-10 minutes away from San Pedro. Nurse sharks seemed to be everywhere; I suspect that some of the dive ops do a little chumming, so the nurse sharks hang out near the various dive sites. Generally little current.
In terms of comparisons, I do prefer Cozumel. I find the coral structures at the deeper sites in Cozumel to be more dramatic, and the shallower sites in Cozumel have more abundant life. Not that it's bad in Belize; I just think Cozumel is better. In terms of the towns, Cozumel is a cruise ship destination, with the good and bad that entails. It means more development, and more people, at least during the day. I would usually take a nap in the afternoon, and by the time I went out, most of the cruise ship passengers were gone or heading back to their ships. Cozumel has some great restaurants. San Pedro in Belize is smaller, much less developed, more of a developing-country feel to it. The restaurants I ate in were usually ok-good, but I didn't hit anyplace I'd call great.
I think either is a great location. If depth isn't an issue, I would probably pick Cozumel, although a good airfare would probably convince me to do Belize instead.
