When, where and for how long will you be in Mexico? What's your style of travel and at which price-point? How many of you are traveling together, and will you be paying for anyone but yourself? Which is your home country? Do you currently have a bank account or accounts and what have you learned from your bank(s) about fees they and others will charge you for using an ATM or credit card in a foreign country?
The national currency is the Peso, and in some destinations USD are accepted ... Pesos are probably what you should be using. We see comments posted to discussions here at TTMB that It's becoming difficult for many visitors to exchange USD for Pesos. The widely-held belief based on the experiences of frequent travelers to Mexico is that obtaining Pesos via an ATM is preferable because of exchange rate and 24-hr. convenience.
Relatively few ATM machines in the country dispense USD. Larger and more expensive hotels tend to accept credit cards, as are many restaurants. Pesos are the currency for most small restaurants, hotels, markets, etc. Airline's, auto rental companies and bus transport companies accept credit cards.
LW