On the Oneworld side, at least for South America, using AAdvantage miles for the itineraries you're talking about will probably be a significantly better option in terms of total miles required than using BA Avios (regardless of which oneworld carriers actually flown).

BA now charges per segment (SF to MDZ is going to be at least 3, maybe 4 separate segments), whereas AA should book all segments from origin to destination (with a stopover at one gateway city if desired along the way) for something roughly comparable to what BA now charges for a single flight having no connections.

Due in part to matters of availability, I often still book F using AA flights from SFO via JFK (with a stopover) to EZE, which is 62,500 AAdvantage miles each way. JFK-EZE on AA using BA miles would be 75,000 Avios with no other flights.

Your choice in paid flights will make a difference, though, to the decision as well. I don't fly much compared to some, but my paid flights are always business or above. I'd have to fly much more and spend a fortune as a result (in J and F) to maintain status on AA, whereas BA's Tier Point system makes it relatively easy for me to maintain a high enough status that I put up with the lower value of resulting miles, since I eventually earn more miles thanks to the bonuses, and all oneworld carriers take better care of me along the way as a result.