i have dove in the keys a number of times . key largo is very nice, dives are shallow on reefs , deep stuff on wrecks. reefs 25-30 feet, due to shallow, longer dives , good light for photo and diving, i like amy slates dive operation, also have dove with horizon divers there they are excellent also. depending on conditions , i would dive whatever based on best conditions of the day.
also dove in marathon with bill and heidi at abyss dive center which was at holiday inn. now bill and heidi moved to their own motel in marathon, nice operation, dives there can be shallow or deeper , reefs up to 60 -70 feet but most shallower.
visability both places varies on the wind , at times green water if it blows out of gulf could boe 20-40 foot. when the blue water of the atlantic dominates vis can be 100 ft +. i have seen it change overnight. also in one location green and another blue. thats my read. have not been to looe key.