Definitely do Caye Caulker for 2-3 days. We just spent a week in San Pedro and were amazed that all the same trips/activities were offered on CC for much less than AC. CC is much more relaxed and there are a number of reasonable restaurants and bars. More than 3 days may get a bit much on CC, but then you could take the short boat ride over to San Pedro and stay there for a couple of days as well.