The weather is usually one of the major deciding factors between Samui or Phuket given they have differing seasons. Mid November through to March early April I would choose Phuket when the seas become calmer and clear and the days hot and sunny...we usually visit Phuket in Jan Feb or March...never seen rain or even a cloudy day.

If your travelling in our school summer holidays then Koh Samui might be the better choice weatherwise. Of the beaches on Samui Cheong Mon is among my favourites (where the Imperial Boathouse is based). Its quieter than Chaweng but near enough to visit of an evening for a meal and shopping.

Both islands have access to diving sites but I know little about these.

Of the islands Phuket is the more developed, Karon Beach is an excellent beach, Karon the resort quite lively but not as busy as Patong.

On a personal note I prefer Phuket, but I usually tie it in with nearby Krabi as well, but the argument of which is best has been raging for a while and to be honest both islands are great.

Kazzy, we went to boathouse March 4th - weather was fantastic, so was the hotel and can't wait to go again.