Both MC and Visa has system charges of 1%, American Express is currently 2.7%. However, the actual settlement rate differs markedly country by country. American Express settles, allegedly, once per day at a wholesale rate in each currency. Visa and MC procedures vary by currency.

A couple of years ago in a controlled experiment the net cost of the FX transaction tended to slightly favor American Express in Euro and most Asian Currencies. Latin American varied widely, but slightly favored Visa in several countries.

For the issuers that tack on their own supplements to MC and V rates any other card is normally better. Remember the American Express 2.7% is not the equivalent of 2.7% added to a MC or V due to the added 1% by MC and V, and the difference in settlement procedures.

Netting all that, the actual typical difference between the three in cost to the consumer is not much different unless your bank adds charges above the MC and V rates. More and more will be doing that this year as other sources of extra fees become more restricted by regulation.

Watch very carefully for over-the-limit, cash advance and late payment fees. They'll eat any reduced FX rates very quickly. If you take cash take it from an ATM that has no interbank fees for your debit card. There are country-specific indices of fee-free ATM's.