I'll stick my neck out a bit. I know it's a drive, but early in the morning there is virtually no traffic so it's an easy ride. When I stayed in Islamorada at the Hampton Inn (had a very nice suite), we made the drive back to Key Largo to dive with SilentWorld. If you plan on doing lot's of dives, you can't beat their pricing. Period. Nor their folks. Nor the dive sites that only they will take you to (Carysfort Lighthouse).
But, Conch Republic will take you to some deep wreck dives (Eagle) and more southern reefs (Secret Garden) that Silent World won't take you to.