You might want to determine the current diving conditions near each of those locations. Earlier this year, visibility was not excellent in general on Thailand's East coast- though Phi Phi was actually a bit better than other locations. Off the West coast, the area around Samui - or Ko Tao in particular - might be a better bet. As for the islands themselves - both are amazing, each with its own unique ambiance. Not sure what the rainfall comparison would be for August.

http://wikitravel.org/en/Ko_Samui
http://www.travelfish.org/location/t...thani/ko_samui