Ko Pha Ngan VS Ko Chang

This might help you decide.

I was on Koh Phangan a week and half ago, was mostly fine weather bit of showers and cloudy days here and hee, nothing extreme, still plenty of beach time with sunshine.. not sure what it is like now through to Sept..

Ko Chang not sure.... but I am also would be interested to know.

I've read in several places that Ko Chang is more likely to have unsettled weather due mainly to the huge concentration of untouched rain forest on the island.. Having been there, I could say this is relatively true.. I was there in March for a period of about 3 weeks.. the weather was great, don't get me wrong, it was just volatile and there were just masses of cloud that hovered for periods during the day over the foresty parts of the island..

http://www.ekohchang.com/geography.htm

In an average year Ko Chang around now is statistically one of the wettest places in Thailand (although still usually okay for a holiday), Ko Phangan one of the drier (Phangan's usual wet season is different to most areas - Oct into Jan). Lots of info, stats in the second link below.

Thanks guys!!!