You might take a look at this thread. My experience was that using the ATM was slightly cheaper. Around $10. US difference for my entire trip, though, so not a big deal either way.
http://negril.com/forum/showthread.php/15716-ATM-Fees
People talk all the time about exchange rates, but the fact is that you are never going to do better than the interbank rate plus a modest fee. Whether the fee is reflected in an exchange rate with a bit of a spread from the interbank rate (as cambios all do) or whether you see the fee as a direct charge, it's in there somewhere.
Personally, I avoid using a credit card when I travel because I got tired of the reissue that occurs when I get home and someone is still trying to make charges to my account.
And I also am uncomfortable asking for a discount when the financial balance is so much in my favor. But we've been around and around on that topic many times.





