6 hours is not enough time. The evening US departures make the airport a zoo - paying departure tax booth (which you will have to pay if you enter Argentina), outbound migration booth, security, etc... traffic etc etc - I wouldn't do it. Figure an hour from landing EZE to in the city in a taxi (there shouldn't be any/much immigration issue at that hour), and figure leaving the city at least 3 hours prior to your departure.

On the other hand, the gate area of EZE is a pit, and the lounges aren't all that great either (as in terribly overcrowded for the hour that US departures typically leave).