I stayed on Pandan Island in March this year.
Some really good dives around Pandan itself and they organise great trips to Apo Marine Park and the Coron wrecks which I'd highly recommend doing while you're there. Both the resort and the dive-shop are profesional, friendly and well-run. Even though they were full, we never had more than four people in any one dive group. A full 3mm suit would be fine. As to the best time of year I really don't know. You'd be best to give them a call. While I was there conditions were perfect. They seem to be booked out most of the time so I'd make a reservation in advance (it's a rough trip to get to Pandan by land and we saw several people arrive without reservations only to be turned away because they were full).
I believe the Lonely Planet described Pandan as "as close to a slice of tropical heaven as you can get". They're exactly right!
I'm more than happy to answer any questions you may have.
Morty