I think Koh Samui is much prettier than Phuket and, notwithstanding recent development, I'm sure it's still more laidback. The main beach, Chaweng, is where all the development is focused. But it still had a nice vibe to it when we were there, especially in the evening, when the beach is converted into a strip of makeshift restaurants, serving decent Thai food and beer by candle light while you hang out on beach blankets and listen to lounge music. And the rest of the island is much less developed and more naturally scenic. With the arrival of some new and better hotels, I'd now recommend it over Phuket unless you're really into the club/bar scene.