As far as i can tell there would be less rain around Koh Samui or Koh Phangan?

It's Thailand some days it rains some days it don't.

Take a chance Don.

The way things have been going this year, you would be brave to predict, even as far as next month.

Statistically both places will have a lot of rain.

October is THE wet month about everywhere in Thailand.

If it helps: we have an extremely dry year.

This can continue in October or all the missing rain comes in October.

In the latter case you would be soaked.

On these charts you see the extreme drought (middle pic):

http://www.arcims.tmd.go.th/dailydata/yearRain.php

Looks like the southern east coast (Samui) was dryer then the west coast.

So much from the glass bowl

I used to think rains started late April, and October was a month of continuous drizzle (rather than heavy flash floods). It's changing though, so hard to predict these last several years.

Edit: I'm referring to the North East in the above text.

Edited September 22, 201510 yr by Shiver

It can be very variable down here at Pranburi literally within a few miles , Id say we had more rain this year than last, but two years ago was VERY dry

I remember a hotel advertisement (not in Thailand):

20% discount if more then 6 hrs rain between 06:00 and 18:00 hrs.

Suggest it while making your booking.

Ko Samet traditionally has the least amount of rain annually in Thailand.

Good question because it's the time of year rain can overlap in both areas. So don't roll the dice and book too far in advance. Wait until you are in Thailand and a few days before you go check the weather forecasts and choose based on that.