If the trip is not until after Oct 1, I would wait prior to making the decision. AA/BA/IB will be announcing join-routes due to the recent authorization on ATI and it is likely the FF schemes may get some tweaking as well.

Seems like these people are not consistently flying the same airline. In that case I would recommend AA since it has probably the best earn/burn ratio generally (although its not the best for some geographies). You do need to fly 4 segments on AA per year to gain status according to their terms but by my parents' experience, this is not enforced. My parents have flying only RJ and BA and made it to Platinum on AA.