Howdy, Rob! We have been to Little Cayman/Cayman Brac at times from late April till early October. We like (no, make that LOVE) warm water, and you'll get 80-85 during those times. The viz will usually be 75-100 feet (typically 80 feet) due to all of the spawning micro-organisms in the water. I have heard a couple of locals say that the weather can be iffy in the winter, with rough seas making diving more difficult. The tradeoff is better viz in the winter.
Personally, our favorite time is September, with water temperatures 85-86, although there is more risk of a hurricane then.
The "Sister Islands" are a pretty good mix of pelagic and macro subjects, although if I was seriously into big fish, I would go to the Bahamas. Other than nurse sharks, we don't see too many sharks, although on the recent trip, we saw a reef shark and a lemon shark. Southern stingrays are very common, and we often see spotted eagle rays. Manta rays have been seen, but they are not common. Nassau groupers and barracuda are huge. Turtles are common, and you'll see them almost every dive. Lobsters are huge, too, and you'll see them on most dives, even walking around during the day. Maco subjects also abound, and I had my 105mm lens on the camera for half the dives.
What attracts us to the Sister Islands are three things:
1. They're easy to get to. Island Air flies into both islands from Grand Cayman several times a day. It's only a forty-minute flight in a Dehavilland Twin Otter.
2. Both islands are still virtually undiscovered. There are no crowds, no hustle-bustle, and virtually no crime (at Southern Cross, they give you a key, but they also tell you there's no reason to lock the door). It's very quiet and peaceful.
3. Diving is absolutely world-class, the best we've seen in the Caribbean. The coral reefs are unbelievably robust and pristine. Even the best sites don't get remotely the amount of dive pressure of sites on other more touristed locations.
On you list of MUST DO dives, Bloody Bay on Little Cayman should be at the top of the list. It's truly amazing to be in twenty feet of water and suddenly look over a vertical precipice that drops into several thousand feet of water!
I'll stop raving before I start sounding like the Chamber of Commerce! If you have any other questions, feel free to email me.
Jim