Making blanket statements about "use issuer X" is a poor idea; the discrepancy between rates differs significantly depending on your currency. Sometime after my last trips to Japan and India, I did research and discovered that V was better in Japan and MC was (.1%) better in India. A later trip, V was better in India than MC. I think on average it all evens out in the wash, but if your international travel is restricted to one country that's always better, then obviously you should always care about that.