I've asked a similar question..the answer is that it's variable.
Samui gets statistically slightly less than Phuket...although Phuket can be wonderful in May and the prices are much cheaper.
Chiang Mai is very humid and you may get storms, though probably not continuous rain.

It is weather though....if you can get to BKK and then decide it would be ideal.....

http://welovephuket.blogspot.com/2007/06/wet-season-in-phuket-great-time-to.html

This is a good read...there are links to a Samui weather blog as well.