Those are three votes for Thailand, and I can add a fourth, and of course I would encourage you to come to Phuket, where I work, as part of your tour.
However, you are looking at quite a long trip, and I would not necessarily recommend that you spend it all in one place. So...
1. If you can come in April to Thailand, you might, might, still be able to get on a liveaboard to the Similans, which is well worth it. If you come later, you will almost certainly not be able to do that as the park closes and the sea conditions deteriorate. I'd say to do the LOB up here first, without undertaking any courses--just fun diving.
2. If you like it here on the Andaman side of Thailand, consider doing your DM training while in the area and then continue your travels.
3. If, however, you're ready to sample different areas, you could move over to the Gulf of Thailand to do a little diving and check out options for DM training there. They will be in the beginning of their high season at that time, when the sea conditions are at their best.
4. Moving south, Malaysia's east coast will be in season, and the South China Sea dive sites of the Terengganu Marine Park are worth diving. Don't forget Borneo and the world-class Sipadan area. DM training at the Perhentians or Redang is enjoyable.
5. Whenever I have the time and money, I always go to Indonesia to dive. You shouldn't miss it. It's too hard to pick a favorite place, but they are all pretty much accessible at the time of year you will be here: Komodo, Lembeh/Manado, Bali, etc. are all easily accessed though Kuala Lumpur and Singapore.
