We just got back from EZE in January. We used the Aeroplan site to search for Star award space and then give that info to the US agent over the phone. Agent had to make separate one way awards to get it to work, 60k miles pp in total. UA from EWR to EZE. Flight was less than half full.

Both cities are great choices. Exchange rate in BA is awesome right now but Rio is just amazing. Really depends on what you're looking to do or see. SCL is not really worth making that kind of trip in a weekend. The best of Chile is outside of the city and you wouldn't be able to get anywhere in that short of a time span.