Hi there,
If it was me (and it will be this april, and I am), I´d head to PG before Bohol, but they´re both great places, so much of a muchness. I´ve never been to the Similans so I´ll leave that choice to you.
One big plus for PG is that all the sites are within 10 mins boat ride, so back at the restaurant for mango shakes between dives...nice. Most of the sites in Bohol are close too...but some of the best ones are a trip away, like Balicasag and Cabilao.
I think PG has a bit more variety too....strong current in the canyons; messing about inside a wreck with the Alma Jane; sharks on demand at shark cave; and the best critters I have seen eg flamboyant cuttlefish; ornate pipefish; tons of nudis; and if you can get a day trip to Verde island organised, that´d be well nice.
The above-wave vibes are quite different too...bohol seemed to me to be more laid back tropically chillaxed, with nice beaches and sunsets; while PG has terrible beaches but more good restaurants, bars and pool tables etc.
It would be nice to hear from someone who prefers Bohol....because admittedly my time was short in Bohol, only about one week.
One bonus about bohol would be that you are within striking distance of Apo Island, which has some superb diving too. Just get the 2-3hour ferry from TagB to Dumaguete, then down to dauin and across to Apo. One pro for PG might be that it has some fantastic tech schools, if you´re that way inclined...techasia are my choice
Either way...should be the nuts,
Ferg
PS if you want some more info re who and where let me know....cheers...