AA if you are purely focussed on earn/ burn and quick 2nd tier via challenge (Exec Plat is harder) and ability to upgrade AA fares
QF for the quickest way to 1st tier on Biz fares, global lounge access (inc in US), although note the min 4 segments. Also has a lifetime 2nd tier qualification which may be worth pushing for.
If you are flying enough I would consider going for both. QF so you get access to the AA lounges when flying domestically, and AA so you can get eVIPs to use on your AA Y-class fares.