Just did some transactions in Singapore and Bali.
At SIN, I did two transactions at Shilla, back to back. They are:
1) Barclay A+, S$68, pending $48.25 (1.40933), posted $48.70 (1.39630)
2) Amex SPG Business, S$253.9, pending N/A, posted $181.07 (1.40222)
Apparently, A+ uses posting date's rate, and presumably Amex uses the pending date's rate (pending amount wasn't shown on its website but I remember making some calculations right after the transaction using available credit and concluding that the pending rate was also 1.40222).
I remember seeing somewhere that MC uses transaction date's rate, but seems like I saw that while dreaming...
Also some transactions in Bali:
1) Amex SPG Business, IDR 645000, pending N/A, posted $47.41 (13604.7)
2) Citi Premier, IDR 271040, pending $19.89 (13626.9), posted $19.93 (13599.6)
These two transactions happened within 30 minutes in the Bali Collection.
Even though these transactions are not "back-to-back, on the same machine", I do feel that this shows something useful, as travelers are not gonna always do transactions on the same machines. Using data points of the same region within a small time period can more precisely show the possible results travelers can get.
Here, Amex looks better a little bit.