Which is better: an international wire transfer or ACH?

If you are doing this from the US it is the same. It's just nomenclature. International ACH is basically providers who have direct credit facility to various countries and their banking systems. If your model for transactions is based on good funds and then settled the International ACH would work if the payouts are many. If you only have a single payout, the wire transfer would be better option. Really depends on your needs and transaction volumes.