It depends on the country. In the big ones, Amex in my experience tends to be a little bit worse than MC, but it's small (maybe 0.1%) and could be just noise. People complained about Amex in this regard much more in the past. In the smaller ones, I've gotten some horrific rates from Amex. Off by 3-5% from what MC was offering. I can't say if they've changed their behavior since then as I've stopped using Amex for charges outside Canada/Western Europe.