If this move to Pto Varas is permanent, then I'd say go with LATAM, since you'll be flying them almost exclusively to/from PMC (stay away from Sky!), and over time alliances/partnerships can change and you may find that AAdvantage isn't beneficial while living in Chile.
Ultimately the choice is based on how you will use the program - free flights in Chile? Free international flights? Upgrades on long-hauls? In my time as Comodoro Black I found LANPass to be much less rewarding for me than AA, and I switched to AA. But that's me - everyone travels for different reasons and in different ways. That said, my time living in Chile is temporary, so AAdvantage makes sense for me knowing that I'll be living back in the USA eventually.