Diving in Coz is what triggered us to look into Belize many years ago. Now I go to Belize every year and haven't been back to Coz since. Loved the drift diving but it was just too crowded for us.
Where you would go in Belize would depend on your priorities. If you just want to dive your brains out--nothing wrong with that--then I'd say head for one of the resorts out on the cayes. Turneffe Island Resort, Turneffe Flats, Hatchet Caye and Blackbird Resort come to mind. Eat, sleep, dive. Think summer camp for adults. Expensive but inclusive with very nice accommodations, good food and lots of diving.
Most folks go to Ambergris Caye followed by Caye Caulker. Different islands with somewhat different vibes. Lots of dive ops to choose from on AC, along with accommodations, places to eat and drink.
If you want to spend any time checking out the mainland, then look at Hamansi in Hopkins or, my favorite, Placencia, both of which are further south and border the jungle. Accommodations in Placencia range from the high end, like Francis Ford Coppola's Turtle Inn, to youth hostels and all points in between. I always dive with my good friends at Splash Dive Center.
ReefCI looks really cool. The lack of AC would give me pause, too. I don't really need it during the day but man, it's hard to sleep at night without it (personal experience out in the jungle). Hopefully they have some big box fans.
Belize is really easy to get around by air or car. There are two puddle jumpers, Maya and Tropic, with no real difference between the two. They can get you any where you decide to go except for the atoll resorts. If you do one of those then you'll be transported by that resort from BZ City and back on a boat. You could also rent a car and drive. There aren't that many roads so getting lost isn't a problem, but I don't really recommend it for first-timers. Btw, you probably don't want to spend any time more than you have to in Belize City. It's pretty rough, not a tourist town like San Miguel. Depending on your schedule, you might have to spend the night there if you're headed for the atoll, in which case I'd recommend the Ft George Radisson or The Great House right across the street.