Usually it doesn’t matter whether you have an ATM card with the Visa or Mastercard symbol. Most places that accept one will accept the other.   However, there are certain countries, usually those a little off the beaten path, where a Visa or MC symbol is favored. In Sierra Leone there was only one bank that took our ATM card with the MC symbol. If their ATM’s were down, or had a line, it could be a long wait to get money – in a country where having cash is an absolute necessity.   Carry both if you can, and if you’re traveling anywhere that’s less than modern, check to see if there’s a preference before you go. (By the way, a Visa symbol is the safer bet in West Africa.)