Koh Phangan is Samui's smaller cousin. The backpacker set began coming here when development started pushing them out of Samui. Now, more and more up-scale resorts are opening on Koh Phangan, so the gap year travelers are being forced to move on once again.
There aren't really any significant dive sites in the immediate area of Koh Phangan, so about the only advantage of basing yourself here instead of Samui is that you might save yourself a half-an-hour or so commuting to the dive sites around Koh Tao.
There don't seem to be many dive operators on Phangan. In fact, the only significant one seems to be Phangan Divers, which offers a full range of courses.