It depends how much you are flying, in what classes, and what benefits are important to you. AA has lifetime status (as does QF I think) which may be useful to you. If you happen to be based in the US you can get a credit card which gives AA miles towards lifetime status as well.

But the earning rates are different on the various combinations of carrier and FFP in economy, and some have status bonuses and some don't, and some do only on some airlines.

So there really isn't a simple answer to this, unless you are only flying a few flights, in which case it definitely makes sense to pick ione programme and put everything there so that at least you get some useable amount of miles at the end rather than an unusably small number in each of multiple programmes.