In my and my friends' tests of various cars overseas, there has been almost no variation between the underlying rates charged by different banks using the same network. Most seem to rely on the Visa/MC/Amex networks to do the conversion (rather than the banks or card issuers themselves doing the conversion) and rates tend not to vary much even between the different networks. Our most recent head-to-head comparisons suggested that Citigold products (using Mastercard/Cirrus) beat various Visa/Plus products (TD Bank, Schwab, etc.) slightly, but we're talking fractions of a percent and I'm not sure that you can replicate the result from one day to the next or from one country to the next.