mineok65 and cunning
Many questions to answer - will answer a few.
Rental assistance.
Once you are past the first 2 years (as an SO) you get what is called the Rent Free Zone(RFZ). This amount will give you a reasonable standard but not great. If you choose to use this money to buy, you will need to downgrade your standards in order to pay off your flat in a reasonable time frame. Or, just like Sydney, you can go to the outer suburbs and get a house but with the appropriate reduced potential capital gain and long commute time!
Its a bit difficult to quantify but assuming you are just renting in HKG, then you will come out ahead as you can live 'rent free' in HKG and buy the house/flat you want in OZ and rent it out.
Education.
Not as good as it looks. In the blurb it says you get 90% of ESF rate(or 75% of other schools)...sounds good. BUT, I would say the majority of CX expats have their kids in International Schools where the fees are significantly higher. CX will pay up to 75% of these fees but the amount is taxed. To keep it simple assume you will pay about 1/3rd of school fees (after tax) which will be around $3-5K AUD(that you will pay) for primary school.
Basings.
If you want the backyard etc then you may end up wanting an Aussie base. It is hard to predict the effects of our changing retirement age on basing availability but I suspect it will make them harder to get. On a base, on the UFO scale, you will be on approx $94K before tax. Eventually that will rise up to $140K'ish after 4-5 years on the base. Obviously these salaries are fully taxed in Oz and you do NOT get any expat benefits.
Command
Its hard to predict...right now it is about 13 years in QF and 9 years in CX. QF already has RA65(via domestic fleet) and CX is moving towards RA65. My best guess is that command(with RA65) will be at 12-13 year mark in CX unless they ramp up the expansion massively - and no external shocks like SARS, 9-11, asian contagion, bird flu etc etc.
Career earnings?
Good question - if I had joined QF when I joined CX I would be earning more but I would be paying Aussie tax. Cost of living in HKG with kids is much higher than Oz. If I went on a base now I would definitely be on less than I would be on in QF right now. Thats the past, the future.....?
There are many intangibles that cannot be numerated. Such as the ability to bid across aircraft. In CX there is minimal control over that. Roster bidding is in its nascent stage in CX. Someone of my seniority in QF would be on about $70-90K AUD more a year than I am on now as an A scaler in HKG.
Bottom line - apply to both. If both offer you a job then you can decide what is important to you.