Hi,
I have just returned from 3 months in Thailand, and did lots of diving and traveling around.
Really it all depends on what else you want out of your trip. How long you are going for and what your budget is.
In order to dive the Similian Islands (rated amongst best dive sites in the world) you have to go on a liveaboard as the islands are too far away from the mainland for day trips, and there is no accomodation. A 4day/3night trip will cost you around 15,000BHT which works out at around £230 per person, all food included, rental of gear may be additional, however most places offer discounts if you bring your own.
Hin Muang and Hin Daeng are accessible by day trips from Koh Lanta or Krabi. Most of the dive shops offer a 3 day package at around £120 per person. If you are staying on Koh Lanta head south as the beaches are very busy, very loud (parties going on until 5am) and way over priced. We did not actually dive here, we spent a whole day (longer than it took to get from UK to Thailand) travelling from Koh Phangan (on the gulf coast) to Koh Lanta (on the andaman coast) and we were really disappointed, everything twice the price as the gulf coast, and not good value for money. We left before we dived and returned to Koh Tao, which would be my recommendation. We dived with reef-rider -
www.reef-riders.com. they are a small school, very professional, and know the dive sites like the backs of their hands. Small groups and an absolute pleasure to dive with. Here you can make a dive for around £10.50 (half the price than Andaman coast), less if you take your own equipment. Chumphon (we dived with grey reef sharks here) and Southwest Pinnacle are top notch and the best for regular day trips, however if you can get to sail rock this is a fantastic dive site. We spoke to people who had dived Hin Muang and they were really disappointed with it, and much preferred Koh Tao. There is stunning scenery on the island, and you are spoilt for choice with excellent snorkelling bays - you can snorkel with grey tip reef sharks in one of them, guaranteed. I will stop whittering on about it now, god I wish I was back there.
Hope you enjoy wherever you end up anyway.