I can't compare as I have only been to Coz, but the diving does not suck. The coral is huge and the fish life is fantastic. It has turtles, rays, eels, lots and lots of tropicals and the terrain is incredible. I did note that the dive sites were in the 80ft plus range and seemed dark even with the sun out. The visibility was 80 to 100 plus in most areas, except the wreck (terrible vis due to run-off). I dove, dove, dove in Coz so can't tell you what else to do besides eat. The dive sites are close within 30+ mins from the cruise dock. My favourite sites were the Palancar Caves and Gardens and Barracuda.
Have fun!
Oh, do some research on Scubaboard.com whether you dive Belize highly depends on what area you will be docking in. Coz is one island, has two docks and both are close to diving, Belize has many islands.
If this is your first cruise, watch out for the drink prices they can really add up!