I saw someone doing the "shark dive" at SW on our last walk through. IMHO you are quite restricted in the cage and there isn't a huge amount of space in the shark tank. If sharks are definately your bag then I think it's going to be a memorable experience, however if the diving aspect is what is attractive I would consider the scuba experience in the living seas at Epcot. For me the attraction of scuba diving is the freedom and weightlessness of being under the water with the animals, this is much more of a part of the experience at Epcot than at SW.
From watching both offerings, I would prefer the Epcot experience.