You should bring both. It is easiest to bring US Dollars with you and exchange locally into Peruvian Soles. There are many exchange bureaus where you can change your Dollars. You can exchange your Dollars in Banks but you will get a much lower exchange rate. There are many ATMs in Peru and they all dispense US Dollars and Peruvian Soles. Credit or debit cards are also easy to use at most establishments. Be sure to check with your bank prior to your departure to inform them that you will be using your card in a foreign country. Traveler’s checks are less efficient and may take extra time and documentation to use. Find current exchange rates.