for a short trip like that, I'd rather visit 2 cities, like BA and ? - would pretty much depend on availability of reward tix with non-stops (in-bound will likely be a redeye and off to work).
UA has avaibility to and from EZE, but not from MVD or GIG. I should have enough AA avios to use one-way redemptions. Non-stops are a must, but a stop-over would probably be a better idea.