As Quero suggests, check the season first and then make your plans.
East coast of Malaysian mainland is good April-September, Borneo is another matter altogether and access to Sipadan from Singapore will require at least 2 flights SIN-KUL-TWU or SIN-BKI-TWU, although I guess you could take the bus / train to KUL from Singapore.
Personally I have not dived in Thailand since the mid 90s, but have dived a lot in Malaysia. Forget Langkawi and the west coast, go to Thailand if planning to travel from October-March, and east coast of Malaysia April-September.
East coast of Malaysian mainland is good April-September, Borneo is another matter altogether and access to Sipadan from Singapore will require at least 2 flights SIN-KUL-TWU or SIN-BKI-TWU, although I guess you could take the bus / train to KUL from Singapore.
Personally I have not dived in Thailand since the mid 90s, but have dived a lot in Malaysia. Forget Langkawi and the west coast, go to Thailand if planning to travel from October-March, and east coast of Malaysia April-September.