There are two currencies accepted in Cancun, US dollars and Mexican pesos. If you have to choose one, my advice is to choose weights. However, there are some situations where US dollars work better for you. Credit cards such as Visa, MasterCard, American Express and Discovery are widely accepted at most major Cancun resorts, restaurants and tour operators.
While most hotels, stores, restaurants and tour companies in Cancun accept payment in U.S. dollars, smaller merchants, stores and street vendors can only accept Mexican pesos. On the other hand, if you plan to enjoy a lot of tours and activities, you'll find that the prices in Cancun are significantly high.