The idea of leaving the luggage at AEP seems to make plenty of sense. It's definitely close enough to the city that it won't be expensive to take a extra cab back there to get your luggage. I would just check the time that the facility is open until. I know that EZE operates at all kinds of crazy hours of the night, but I personally don't know if AEP is open as late. With that much time though, you definitely should enjoy the city.