Koh Samui is probably the best decision by far. If you had a longer trip Bali is nice too, but the previous poster had a point regarding the travel time. Another thing to think about is that Koh Samui may be feeling some serious effects as well. If not water related perhaps social issues. Refugees from the west.. etc etc.. Might be socially interesting. You might even be able to volunteer some time to help those in need. Some might think that would be really neat, but then again some might think that was a far cry from a beach so YMMV.