I would think it depends on what you ultimately want to use the miles for - IMHO if you are simply going to take free trips on the same route you travel, it only seems to matter that you accrue on a single carrier to consolidate - but for ease on other routes - say NRT or LHR, might make more sense to go to AA (as opposed to a partner) - but, for instance, if you also use Amex/MR, AA is not a partner, so that would factor in as well - just what is the best scheme to get the most miles in a single account, gain status, and ultimately get miles you can use! Congrats on the Mexico purchase - such a great country.