First good luck on your future adventure. My wife and I moved to Caye Caulker one year ago and we have loved our adventure.
You can contact us at our website (and learn about what we have discovered) at www.goslowbelize.com
We will be happy to answer any questions you may have.
You can get a feel for Caye Caulker in 3 days but I don't think you can see everywhere in that time.
Diving from either island is the same experience. Both islands use the same locations.
I think the best place to meet expats on Caye Caulker is Friday night at the "Jam" or Wednesday night at Luciana's place.
We hope we get to meet you when you're on the island.