Exchange rates for ATM are not that different, it is like driving all over the place to save $1 while spending $5 on fuel.
I travel extensively and normally just use a credit card. Primarily for the consumer protection if you are using cash if it is taken it is gone. If someone takes your card number Bank America places a fraud alert and supplies you a new card. The only down side are the foreign transaction fees. Capital One does not have the foreign transaction fees, but still has the fraud protection, just not the miles on my Airline.
Down side of the ATM card it is cash, in Mexico people are sometimes forced to withdraw to the max limit before being released.