I have to put in my vote for the Cayman sister islands. We have been to Brac twice and we are going to Little Cayman this Sept. We also have been to Coz, and ZF2NT is correct. We were spoiled before we got there. I think I am one of the few people I am aware of who didn't really care for the diving in Coz. It may have been our operator taking us to bad sites or something, but I never had a single dive on Coz out of 15 dives that was better than my worst dive on Brac or Little Cayman. Speaking of operators, I have never seen any operators as thoroughly safety conscious and knowledgable about safe diving as those we experienced in the Caymans. I don't doubt that such operations exist on Coz, but our experience there did not include one.
To me the biggest difference is the lack of current (for the most part) in the Caymans. I know most people love the "relaxing" diving they get drifting with the current in Coz. However, I am one who loves to sit for long times exploring smaller parts of the reef, so having to work to get out of the current to do so, and having the pressure of keeping up with the rest of the group didn't suit my diving style well at all in Coz. Nearly all of our dives on Brac and LC were moored dives with little or no current, so you were free to head out in whatever direction suited you, and had no pressure to follow anybody or to keep up with a group. I also think the health of the Cozumel reefs just wasn't as good as what we saw in the Caymans. In the end, we could probably do more frequent cheaper trips to Coz, but if we hold out for an extra couple of months, we can afford to do the Caymans, knowing that we will both enjoy it alot more. Factor in the extra costs of taxis, meals eaten out, etc, and Coz isn't necessarily that much cheaper if you go with comparable accomodations and dive operators.
Enjoy your trip to Coz, but take the time to go to the Sister Islands sometime and I guarantee you won't be disappointed.