We dive on both sides of the border and like them for different reason:
Diving in Thailand and Burma is very different. Seeing both is a good idea to see more diversity.
Thailand has better visibility and bigger schools of baracudas and trevalis due to the fact dive sites are in a national parc protected from fishing during the season.
Burma has a lot less diving boat (we are alone on every dive site while Richelieu Rock can have 100 divers at a time), more rare species (frog fish, seahorses, ghost pipe fish are seen on every trip, harlequin shrimps very often, nurse shark and big blotched sting rays quite often too) there are many caves in Burma as the rock is limestone, none in Thailand. There is more current, hence colder water (27-29'), hence coral is in a lot better condition in Myanmar.
Burma also have more challenging dive sites (deeper,current, caves).
So really, while nearby, it is two very different destinations.