I think a lot depends on what you want. The Similans has some great macro while the Maldives has outstanding big animal action. The Maldives is a large area so a lot depends on where you go. Despite bleaching there are still some healthy hard coral areas as well as soft corals and gorgonians. I think the Maldives is maybe better for the sheer number of fish you can see and Similans probably better for soft corals and gorgonians. But if your looking for an adrenalin rush the Maldives is hard to beat. Reef sharks, hammerheads, whale sharks, manta rays, schools of fish, if you like them big its the place to go. Ok Cocos and Galapagos and maybe French Ploynesia might be better (havent dived any of them) but the costs would make the Maldives feel like a budget holiday. In terms of budgets there are decent cheaper liveaboards. Also look out for special offers.
Btw, if your already in India the Andamans are easy to get to. Kinfisher Airlines has cheap flight to Pot Blair, and accommodation and diving is not very expensive. I cant comment on Andaman diving since I have not been there yet. But have a friend who works there who showed me some great photos. Maybe next year...........