We've been diving off of Key Largo and Key West and recommend both. However, Key Largo from a dive standpoint is your better site. Quiessence in Key Largo (adjacent to the marriott) is where my daughter and I just finished three days of diving. They were great (nice people, 6 person boats) and we had our best dives on Molasses and French reefs but there are a ton of good sites. Key largo is also pretty convenient, about an hour from Miami airport (in times of low traffic.) In Key West, the non-diving activities are better (some really top notch restaurants) but there is a great wreck here and some good reefs. I've gone out with a few shops in Key West but would recommend Southpointe Divers dive shop. I think our next trip will include a few days of diving in Key Largo followed by a drive down to Key West to finish up.
Hope you have a great trip.
Hope you have a great trip.