I use both 28 degrees mastercard and CBA plat Amex. Over many transactions and currencies this year, I have not found there to be a significant difference between the exchange rates of the 2 cards.
I will now always preferentially use the CBA Amex now, however, for the following reasons:
- Instant notifications of transactions through App, showing currency and amount (28 deg does not do this)
- QF (or CBA) points earn
- Merchants can not apply or offer DCC at point of sale (spending a lot of time in China, there is still a lot of "involuntary" DCC practiced on mastercards and visa cards here, which can often only be unwound by filing a dispute. This feature alone is a big advantage to me)
- it is easier to pay the bill than 28 deg
The downside is the annual fee, however depending on the volume of transcations this can be offset by the points. The other downside is reduced merchant acceptance.