The locals say that lines at EZE can be very bad but I've never seen this. I've also never seen truly bad traffic between downtown and EZE.

IIRC Aerolinas Argentinas, code AR and a full SkyTeam partner, which I assume is the airline operating your domestic flight, has a daily flight from EZE to JFK that leaves perhaps an hour or two after the DL flight to ATL. It's not a codeshare and generally doesn't appear on delta.dumb. AR does ground handling for DL at EZE and uses the same terminal, so it should be easy to be rebooked in this way if necessary. AR seems good about honoring SkyPriority but they have a reputation for schedule changes, cancelled flights, and other problems. Despite this, my personal experience with them was fine except that they don't have any alcohol on domestic flights, even in their FC cabin, where they sell food.