If I can recall correctly Carnival told us the Stingray City & Island Tour was the popular one. But they also told us that all the tours end up at the Stingray City SandBar. The only difference is what is included in each tour.
We were lucky enough to be the last ones in line for the busses, therefore, the busses filled up and we got our own "private" mini-van (there were only 7 of us). The driver took us on a tour and gave us much info about the Cayman Island and answered any questions that we had. We went to Hell (I wasnt too impressed w/it), but it was fun to send a post card from Hell to someone! We then went to the Turtle Farm (my daughter enjoyed seeing the turtles). After that we went to board the boat to Stingray City. That was fun as well to see the stringrays swimming around! There were many other boats around when we got there, some coming and some going. You swim w/the sting rays and can feed them, snorkel, swim, etc. Rum punch is served on the way back as well as punch. It was fun and we still had time to go shopping afterwards. But the last tender is a 1:15pm and you have to wait in a huge line to get on a tender back to the boat. But I went fast and smoothly.
If your only interested in the Stingrays and snorkeling you might just do the Cockatoo Catamaran tour...mabie it provides more time for snorkeling and swimming w/the stingrays? I think we only had about 45 minutes or so of swimming w/stingrays.
Have a blast!