We've been diving every year for the last 6 years or so. Most of the time out of Ko Lanta, but last year we also did a trip to the Similans. The Similans are certainly good, but not that much better, if at all than the Ko Lanta sites - which stretch from Ko Phi Phi down to Hin Deng/Hin Muang (the Manta sites). 2 weeks ago, our friends spent 10 minutes on a safety stop watching a good 10m whale shark swim around. Ko Ha and Ko Bida have a lot of macro subjects. Night dives on Ko Ha are great. On the Similans, many guides like to drop down to close to 30m and stay deep saying there is some special things to see. In reality, it means short dives for them, and lots of rest.
Ko Lanta is not very touristy, low key, great for families, and very cheap. The season is end November to now. Whale sharks are mostly Feb-April. Have a look at
Thailand diving . They are very experianced, and good.
Similan has a lot more to offer than Ko Lanta has. We dived with Sea Dragon Divecenter in Khao Lak. Great people working there. And we talked with the divemaster and he was fine with shallow dives. He dide´t understand the reason for diving deep on The Similans.
But Lanta has some really amazing dives for sure. Such as Hin Daeng/Muang, and Ko Haa