I have been to all three places and have loved all three for different readons.
Koh Samui is probably more of a beach resort, we like snorkelling and going places and we didn;t find the snorkeeling as good here.
Phuket is much more commercialised but the Kayak tour was fabulous, also did an elehant rip which was very good. We didn't do nay snorkelling here.
Krabi is much smaller and the shopping is not as good as either of the above but I enjoyed taking boats to the various islands, also did an elephant trip and got the see them bathed and got a change to feed them which was really good.
Phi Phi Island is a good trip from either Krabi or Phuket.
I think possibly a week is plenty in any of these places.
It really depends what you enjoy doing other than the beach.
I'm sure where ever you choose you will enjoy.
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