The only fair and consistent way of comparing is to use different cards at the same merchant at the same time, hoping that they will be processed and charged at the same time and appear on statements at the same time. This avoids daily rate variations.
To those of you who know about this stuff, given that several US transactions on the same day come in at a differing FX rate - is the rate determined by the money market rate at the exact time that the transaction hits your Amex account ?
Knowing that, and watching the online FX rates, would give an optimum time for a buying opportunity of MRs etc.- recently there has been a movement of 3 cents or so within the day.
Thanks