I'd say PG is the best option for overall variety, but there more schooling fish at Apo Island. Turtles would probablly be Apo Island, although you're pretty likely to see them at either.
Mactan would be the best option if, erm, it rained and the diving was canceled?
In all seriousness Mactan has okay diving, but I don't think I'll be making the trip from Manila to Mactan for diving again so I definitely wouldn't recommend coming from further afield!
For big fish schools I'd definately agree with Caloy and say Balicasag is worth a visit, the Black Forest dive is one of the best in the PI.
I've not yet been lucky enough to visit there (yet!), but most people would recommend Malapascua, if you're keen on Cebu that would be a better option than Mactan - although I'd say it would be quicker to get to Bohol (Balicasag) than Malapascua from Cebu airport
If you're here for a while I'd recommend trying out two areas, probably out of Malapascua, Apo Island and Bohol (the Balicasag and Cabilao Island sites are amazing, other Bohol sites are so-so to good). All three of these options are easily accessed from Cebu.
Karl