Cayman is one of the most developed places in the Caribbean, and one of the most expensive. The resorts/hotels and dive shops will also be more expensive.
The diving there is good, with very clear water and walls, and health reef, marine life, etc.
Belize city is not very good, and crime ridden. You'd want to stay in one of the cays, such as Ambergris Cay, or Turneoff. The Blue Hole in Belize is famous. The water is also clear, and reef very good. I dove from Ambergris Cay, and it was pretty good, with lots of soft corals, fans, rods, but not overly colorful.
Another possibility is to do a W. Caribbean cruise. Some cruises would do 4 ports, such as Cayman, Belize, Cozumel, Costa Maya or Roatan. All very good diving ports. If you start from Tampa, you can also go north a bit to snorkel with manatees, and drift at Rainbow River.