Hi!
We are travelling next week to Thailand. Tomorrow we need to decide where we are going. We heard that the rainy season in Phuket is sept. and oct., and in Koh Samui oct. and nov.
Can somebody give us some information about the weather? And also, which island is prettier (prettier beaches and nature). Thank you!

I'd opt for the greater Phuket area. By mid-Nov, the rains have mostly moved away. Consider more than just Phuket, but also Krabi, Ko Lanta, Khoa Lak, etc.

Statistically Samui is twice as wet as Phuket in November.

https://www.travelfish.org/weather_fish.php