Caye Caulker would be the better choice, of course!
Dive Shops based in Ambergris Caye do 'pick ups' in Caye Caulker, so you don't have to be staying on Ambergris Caye to do the dive trips.
What is your budget for the week? How many dives do you want to do? ...etc...
Have a good one!
Cheers,
DK
dekuylen@yahoo.com