First of all welcome to AFF.
BNE = Brisbane Airport
HKG = Hong Kong Airport
I'll assume that you want to fly direct. BNE-HKG direct is operated by:
Cathay 6x weekly - Tue, Wed, Thu, Fri, Sat, Sun
Qantas 4x weekly - Tue, Thu, Sat, Sun
Premium Economy isn't available on the direct flight.
You can see flight availability on
SeatCounter - The Booking Class Availability Machine
To figure out what fare class a fare is in, go to
www.americanexpress.com.au/travel and look at the various prices. Generally the more expensive a fare is, the more flexible it is, and the quicker you'll earn status and frequent flyer points.
QF = Qantas
CX = Cathay Pacific
If you are flying on B or Y fares (on QF or CX), register with
AAdvantage. You'll get extra Elite Qualifying Points (EQP), and get status quicker. B fares are much better on AAdvantage, than on QFF.
If you are flying H (on QF or CX) or K (on QF),
AAdvantage is still good.
If you are flying anything lower, stick to
Qantas Frequent Flyer or
Asia Miles.
If you fly B or Y fares, then you'll earn ~13000 EQP per flight. 4 flights will get you 50k EQP and AA Platinum (Oneworld Sapphire), which is almost the same as QANTAS Gold. 8 return flights will get you 100k EQP and AA EXP (Oneworld Emerald), which is almost the same as QANTAS Platinum.
If you're on one of the lower 1.0 EQP/mile fares (H or K) then it'll take you 6 return flights for AA Plat, or 12 for AA EXP.
See these links for Earning on AAdvantage on:
QANTAS
Cathay Pacific
And these links for Earning on Asia Miles on:
Cathay Pacific
QANTAS
One caveat with Qantas Frequent Flyer, is the
poor earning rate on Cathay Pacific flights. See
this section of the T&Cs. If you fly Cathay Pacific, and want to earn 100% of what'd you earn on QANTAS, you have to buy Y fares, which are horribly expensive and often close to the price of a business class seat.
Status is done differently on Cathay Pacific, in the
Marco Polo Club. I don't think that anyone here recommends crediting to CX because of this reason and the high qualification requirements.
Having Oneworld Sapphire will get you in the lounges in BNE and HKG. Emerald will get you in the First Class (F) lounges (in HKG, SYD, etc). Cathay Pacific treats all Oneworld status members well.
So in summary, pick AAdvantage if you buy the more expensive flexible fares on both CX or QF, or just on QF. If you buy the cheaper less flexible fares on QF and are happy to stick to flying QF, pick Qantas Frequent Flyer.
Will you be earning miles solely through flying, or also through credit cards?
I will also quickly mention Lifetime Status.
Getting 1 Million Reedeemable Miles in AAdvantage will get you Lifetime Oneworld Ruby. You need 7000 Status Credits in Qantas Frequent Flyer for Lifetime OW Ruby.
Getting 2 Million Reedeemable Miles in AAdvantage will get you Lifetime Oneworld Sapphire. You need 14000 Status Credits in Qantas Frequent Flyer for Lifetime OW Sapphire.
And then there is also the issue of burn rates, i.e. what your earned points will get you. Most people here feel that an AA mile is worth more than a QF mile, due to AAdvantage's better burn rates.
If you're flying in economy, it is probably easier to get lifetime status in AAdvantage. If you're flying in Business, it is easier to get lifetime status in QFF.