Dollars, of course.
There are exchange houses that will accept pounds, but you can move about easier with dollars.
First rule: NEVER, ever, exchange money on the street. Always look for a Casa de Cambio or visit a bank (Santa Crus in Sos?a if very good). There is a report that the exchange place located at the airport, to the left as you come out of customs is a good place to exchange your first few dollars.
Second one: Never, ever, exchange more than you need for a day or two.
Third rule: Never, ever, flash a wad of money. You might become a target.
Forth rule: Find out where beer is cheapest...it is so good....but it is served ice cold and goes down real smooth....
No more rules! Enjoy yourself.
HB