qucik answer....if you are looking for things like shoppoing, restaurants, bars, etc. and you are ok with decent, but not great diving -then San Pedro is the place for you.
if you are looking for great diving, then staying at Turneffe Atoll is the way to go. only issue there is it is pretty expensive.
I'm pretty sure you can dive Turneffe from San Pedro, but there are limited diver operators that go and it can take quite some time.
If you don't want to afford staying at a place like Turneffe Island Lodge, you can look at a place like Hugh Parkey's Belize Adventure Lodge. It's close enough to Turneffe that you could dive there every day. They also offer trips to the Lighthouse Atoll and the Blue Hole. The accomodations are comfortable, but the dive operation is top notch.