Generally, I agree with the rule of thumb: one mid-tier is better than two lowest-tiers. The one exception *might* be if you have the chance to add AA Gold. Even if you keep *A as your preferred way to travel, having OW as a backup would be nice (same goes vice versa).

Word of caution in the USA: if you are flying on an airline with which you have NO status, and you run into irregular operations, it may be very, VERY difficult to reschedule your flights. (For example, AA will from time to time reject phone calls from non-elite pax, and I imagine they aren't the only ones who do it.) Airline customer service for non-elites is very poor here across the board, whereas even the lowest level of status earns you a separate phone line (that usually gets answered) and use of first/business class lines at the airport.