It is definitely better to pay with Pesos in Argentina. Dollars are often accepted and Euros probably are too (especially at hotels) but you'll get a bad exchange rate if paying directly in one of these currencies.

I think the best thing to do is to take some Euros or Dollars with you (only €200 - €500) but try not to use them except in emergencies. Instead use your debit or credit card to get local cash out at one of the many automatic cash machines.

Always try to pay in the local currency in order to get the best deal.

What about British Pounds? Can they be changed ok?