I've been in Kho Phan gan, that is near Ko Samui, the last year in august and the wether has been quite good, some shower in the afternoon, but not every day!
I went to to kho samui, but a really didn't like it! Is crowded and really fake I left immediately! KP is smaller full of young people, party every night, nice little restaurant the nature is great, well the sea is not really transparent but the place is amazing! Or if you prefer you can go to a smaller beach without noise and people!

Ciao