Please note: I am not advising you to do anything illegal as I am just trying to re-iterate experiences from my past clients traveling to Argentina. You just need to use your own judgement what works the best for you.
Many American tourists visiting Argentina exchange USD to Argentinian pesos not at ATMs, but on the black market. They do so for pragmatic reasons. Let's say in ATM they get 8 pesos for $1 and if you stroll Florida in Buenos Aires you will see people offering you 12 pesos for $1. So many people decide just to get more money for their buck.
Some shops and restaurants have prices in USD $ and you can make your calculations what would be better for you. From what we know $50 bills get the best rate on the black market and they don't want anything lower than $20 bills. You can google 'Blue dollar' . Also there is a twitter account that publishes 'unofficial' rate. Seehttps://twitter.com/dolarblue
And again, if you decide on doing exchange on the black market there are risks that you can be cheated.