OP,you are correct regarding the amount of rainfalls. That being said, I would not make the selection based on that alone. I would rather go to Southern Thailand over Pattaya any day and I have a house near BKK airport which is about 1 hr from Pattaya.

Of all four choices without taking cost into consideration, I would choose Samui - the amount of rain is similar to Krabi or Phuket but the water is clearer at this time. What airlines have you look at for the flights? I would have thought that PG or FD should be cheaper than that.

regards,
drew