Used to be that everybody preferred dollars, but those days are over.
Why? Because of the War on Drugs. Mexican law has made it difficult to use dollars in Mexico. How? Well, they have to pay an added tax when they try to bank over about $1,000.00 U.S. dollars in a month's time. The drug cartels deal in dollars and there lies the problem. The result is that you will get a lousy exchange rate when you use dollars.
The answer to this problem? It's simple. Guesstimate the amount of money you will need. Keep in mind that while your hotel will accept credit cards, most merchants won't.
Go to your local bank in the U.S. and arrange to "buy" your pesos before you come to Mexico. You will get the BEST exchange rate possible. Then you can use your pesos for all of your purchases. Due to the great exchange rate we are getting nowadays you could save 10% easily. It may sound like a hassle but it pays for itself!