The days where Cash gave your huge returns in Argentina are now over, given the Peso has floated. Depending on where you stay and what types of establishments you will visit, most places will accept plastic. It is true that some merchants and service places may offer a discount if you pay in cash, but this is a case by case thing. As an example, its more likely to find places that will not take plastic in San Telmo vs. Palermo o Recoleta.