Personally I prefer Lamai, the water is deeper than Chewang, plus Lamai is cheaper and far easier to get around. Chewang Beach is more scenic but the water seems shallow and you have to walk out a long way. Chewang Noi is very good and so is Choengmon, I prefer the latter two to Chewang. But in my book Lamai is best. If you haven't already booked at Koh Phangan then I recommend Hat Thong Nai Pan Noi. Really lovely beach.