AA good for earn-burn ratios, apart from discount fares. e-vips on AA and of course the Gold (OW ruby) and Plat (OW sapphire) challenge.

QF good for getting highest tier of Plat (OW emerlad) on J/F fares

My personal view is that each carrier typically looks after its own high tiers in preference to OW tiers so you are best with status on your most flown carrier.

If you are a primary *A flyer and this is just the occasional OW trip, my bet would be AA so the points you earn are worth more