There is more of everything on Ambergris. However, since you will be diving nonstop you will most likely be too pooped after diving, or have an early get up for you next day of diving to be looking for much late night entertainment. Ambergris is generally higher priced than Caulker.
Caye Caulker has some really low budget varying to moderately high priced places and good reasonably priced dive shops. It tends to attract the young budget traveler, especially the European backpackers on the Central America circuit. It still has a good selection of restaurants and even one disco.
I guess it boils down to budget and how much nightlife you want. There is air and water taxi service between both so you can visit whichever one you don't stay on.