I did my divemaster cert at Bahama divers this past summer. I go to the bahamas often and I have done both places. This is my oppinion. I think both take divers to good sites. I feel that Stuart Coves is more commercialized seeking to put as many boats out and get as much money as possible. So whatever attitudes that creates I think you will feel that. Bahama divers has a much larger dive shop and store if that interests you. The people working with them are more laid back, still professional, and I think will give you a better overall experience. I had a chance to see them operate over many days from a behind the scenes point of view. I know they were very accomodating to my schedule as well. Thats my oppinion.
Also if you are staying at the atlantis, they are right there. That is a nice benefit especially at the end of the day when you just want to get back to your room. You won't have to take an hour ride back, I say an hour because you have to stop at every other persons hotel as well.
IMHO...