Derffie:
I have not. Hope to try Little Cayman as one of the stops on a live-aboard trip soon, Lord willing and providing. When you do a forum search for Little Cayman, or pretty much anything Cayman, you may just choke on the # of results. But use the more options button, and search by most recent, because things change over time.
On the other hand, finding reports on Grand Turk is a bit more of a chore. I turned up this trip report by Jungho Kim,
Trip Report Grand Turk, Turks and Caicos, that looks like a decently rounded, fairly recent discussion.
One big question I'd ask...how many dives/day? I'm told with LCBR you can get in 2 morning, 1 afternoon, and maybe a little night diving over the week, so at least 3 dives/day, weather cooperating. At a number of dive destinations, 2/day is typical. How much do you want to dive?
Richard.
P.S.: Just fixed a typo. It's 1 afternoon dive at LCBR, not 3. Sorry about the mix up.