Where is better, krabi and phi phi islands or ko samui?

i'm travelling to Thailand this year for three weeks, the first week and a bit will be spent in bangkok and chiang mai, and the last two weeks will be spent south near phuket. we are thinking of spending one week in phuket and another week in either krabi and phi phi islands or ko samui. we would like to go to all three places but it would be far to busy , so which is better krabi or ko samui. we are wanting to go to beaches with beautiful scenery, clear water, white sandy beaches, palm trees, basically a beautiful beach paradise with great scenery. so where should we go and why . sorry if thats confusing , but all answers are appreciated . thanks