My suggestions would be to do all 3 !
Little Cayman is known as being the mnid blower due to Bloody Bay Wall.
on the Brac in my mind the only thing really worth seeing is the MV Capt. Keith Tibbetts's Wreck.
You could definately spend a whole 5 days diving little Cayman and never even come close to running out of dives. However the diving in Grand Cayman is very different and can be just as exciting and in my mind if you are coming down here I would not miss out on a few dives here in Cayman maybe on the East End and Northside.
There are plenty of Cayman regulars and even a few of us that live here so I am sure if you have any direct questions then we can help out more. And of course the best spots are all subjective, but I would definately mix the trip up with all three ! (you can dive the wreck from little cayman, so you only need to stay on 2 islands)