Certainly dollars
You should have some cash, some traveller's cheques and your ATM card.
Change your dollars at an exchange bank or house( Casa de Cambio--often times just called the "Cambio") You should always change just what you will need for a day or so. Lots of cash attracts the wrong element..
Never, ever, exchange money on the street- the hand is faster than the eye....
Enjoy...
HB