COPA is good, but if you would like earn miles, they will loose their SkyTeam membership in October this year. And IMHO they were not invited to join *A, yet.

Have you considered MX (they fly to GUA, SAL, SJO, PTY etc.)? You should be able to credit MX to AAdvantage (plus they will be joining oneworld soon). There are some negative voices on this forum, but I like MX and never hesitate to fly with them (unfortunately not more than once a year these days, as Latin America is not my first business destinaton any longer).