If you are thinking of Lamai beach then look on the web at Lamai Wanta. Don't know if you will manage to get any accomodation there as it is getting very popular.
If you are thinking of Samui for that time of year I would book.
New Years eve is good with loads and loads of fireworks.
some hotels - mainly the larger ones- charge for a new year gala dinner and it is compulsary. Just watch out for that.
You may get some rain but you should get sun too, just go with the flow.
We will be in Cuba this year spent the last 2 in Thailand.
Enjoy.

__________________
Kiss
God laughs at people who make plans.