I have only dove with Horizon, so it is hard to say they are better than any other dive shop; but I have only dove with Horizon because they are a really decent dive operation.
They have onsite gear storage if you are diving multiple days: great to not have a wetsuit dripping all over your accommodations. If I remember correctly they offer free air fills, and Nitrox fills are very affordable.
Finally, I like this dive operation since they let you plan your dive, then dive...I have heard that some other Key Largo dive operations pretty much treat most divers like new divers and micro-manage a bit. Never had this experience with Horizon.