Phuket is a hole. Plain and simple. Party life is great, but really I could only spend a few nights there....DEFINITELY not a week
Koh Samui is much better, but its WAY more relaxed. Dont expect there to be a great night life as its mainly a resort island.
Basically if you want to party - Phuket is ok, but i would say Bangkok is way better
if you want to relax, Koh Samui is awesome... Im going back in January.