We are flying to EZE out of LGA for $765..This may be better than going thru MIA.

Beware, when your are flying back from EZE, if customs at IAD mid-terminal is open, then they will split passengers: connecting ones and their luggage will go to the mid-terminal, clear customs there, re-check baggage and then exit to the C-concourse through a TSA checkpoint. Local passengers will be bussed to the main terminal, clear customs and immigration there and exit to the main terminal.
So, if your bags are tagged to MIA and your plane arrives after 6:00 (or maybe even 5:30?) then your baggage will be automatically routed to the connecting passengers lounge. I am not sure if you can pick your luggage there and exit. You will have to pass it through a TSA inspection point and then carry it back to the main terminal in the "moving lounges".

I will recommend doing this *only* if you do not have checked baggage..If you do, your best bet will be to try to get it tagged at EZE to IAD, not MIA.

Hope this helps