I'll betcha 95% of the time any of the operators will be great. Everyone has their favorites, and probably no one, or nearly no one, has used all the operators down there. Chances are the bad ones are eliminated very quickly.
Look at the variables. Size of boat, price, area of service. There are some really nice reefs that Tavanier charters don't go to because they are too far and there are some nice wrecks that Key Largo operators don't go to for the same reason. And a more expensive dive cost may not mean a better boat or better service (unless you consider snacks and carbonated drinks "service"). Often a higher price is simply an offset of having further to travel to the dive site.
I've dove with three charters off Key Largo, and I prefer SilentWorld. Smallish boats, very reasonably priced when you do mutilple dives with them ($45 for a two-tank dive with air when you commit to 5 two-tank dives), but you'll have to bring your own snacks and carbonated drinks! Have not dove with any charters off Key Largo.
