Weather on andaman coast

I read somewhere that the island Koh Samui is much better for stay within period August - October than places located on west coast(i.e. such localities as Phuket, Krabi, or Trang county) I was on Koh Samui on the second half of September 2001 and I must say that the weather was quite good the problem was only short daily rain between cca. 15.00 - 17.00 o´clock . Therefore I wonder whether the weather on Andaman coast is much worse, whether the rain is stronger and longer in early autumn or whether the situation is still acceptable for common swimming and relaxation on the beach.

Thanks Manicek