Today long story short you don't need to worry about it.
Last year before the economic reforms there was quite a bit going on with the Argentine peso.
Today long story short you don't need to worry about it. Credit card use, is absolutely fine and you wont get charged any extra. Bring the normal amount of USD as a cash reserve in case you have bank issues.
For those interested before Argentina was allowed to buy foreign currency there was a large difference between the government rate, and the free market rate for US dollars. The free market rate which used to be called the Blue Dollar rate was sometimes 40% more favorable than the government rate. Because the inflation rate was quite high the locals did not trust the Argentine Peso. This meant if you brought USD from outside the country and spend them you could get a great deal.
This also meant that if you used an ATM or your credit card you would get charged the lower government exchange and things would be more expensive for you.
Now the Argentine Gov has re enabled the ability to buy foreign currency so everything including inflation has stabilized.
The locals seem to still have a slight preference for USD so some places will give you a 5 or 10% discount if you pay cash but its not the large differential it used to be.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.