Just to add, for prepaid the best value is to find out where the local store for Vietnamese, Indonesian, and Filipino workers are.
If you show your arc and health card, you usually get the sim free, and can buy prepaid cards from Taiwan mobile, chunghua, and FET there for $400nt unlimited 3g. The card will say $500nt but they charge $400 to $420nt.
They give you unlimited 3G.
If you want true unlimited 4G, and can get a contract, they have true unlimited for $1000 a month ($999)