Based on my experience, with the exception of HKG (yes CX is very nice in the first lounge) a lounge is a lounge is a lounge whether business or first. And OW Sapphire gains entry. Though I haven't been through LHR since I became EXP. I'm certain someone will offer an alternative view.

Only if you have enough mileage in the QF bank to be concerned about multiple business class awards SYD-LAX or SYD-LHR and thus have a real use for the Emerald status with QF would I worry about it. OTOH if you never plan on returning to the USA then perhaps AA would be worth a bit less.

Consider the AA platinum level mileage bonus and the relative cost of an AA award on QF domestic versus the cost of the QF award on QF domestic...........

YMMV

Happy wandering

Fred