IF I had to boil it down - the MRT is faster and more efficient, but the bus network does go everywhere. So for me its MRT when I can and combine it with buses to cover the last bit if it isn't walkable.
Trains run frequently between 6 am and midnight, and ticket prices range from 15 THB ( US$ 0.40) to 52 THB ( US$ 1.40) or 140 THB ( US$ 3.80) for a day pass. Ticket machines only accept coins, but the ticket offices will swap your notes for change.