I'm not familiar with Siquijor or Sumilon, so I can't compare. Dauin is very nice, especially if you enjoy macro/ small stuff. Dauin is a black sand beach so it makes for some nice contrast with the fish and coral. I went in end of Aug and the resort (Atlantis) was practically empty! There were at most 2 other guests during my stay there. The main resorts there are bit pricey but worth it IMHO. But it appears that there were more resorts being built there, so prices might be getting more competitive.
Anyways, not sure if there is a peak season in Dauin, but I would highly recommend it as a really quiet and relaxing place. Plus you have Apo Island that could done in a day trip for some variety.
