Unclear, are you wanting to compare Cayman and Coz, or are you trying to compare Key Largo to the Caribbean destinations?
When you take travel schedules, dive schedules, and time-to-fly restrictions into account, it can be hard to get enough diving into a long weekend - I think the schedule has to play a large part in deciding where to go, even though starting from Miami helps a lot. (How long is your long weekend?)
I love snorkeling. But snorkeling in Roatan or anyplace, particularly on any sort of cruise ship excursion or location they set you free for the day, is rarely a good way to judge the diving.