Just returned from HKG, did the following experiment, transaction is done one the same time ( within 1 minute on the same day)

I used BofA ATM card at BA terminal and get HK$500, it costs US$64.52 from my checking account.

Used the CitiBank ATM card at Citi terminal for HK$500, it costs me US$65.34 from my checking account.

To avoid the 3 to 6 % surcharge by Citi and BofA, I used the Charles Shwab visa card, there is no surcharge in China, Hong Kong and Japan.

looks like BofA have a slightly better rate.