It appears that you have an interesting problem......
1. It is only a problem if your are offered jobs at both. To be able to choose either CX or QF with simultaneous job offers is the situation required by only a small minority of pilots.
Generally this situation would arise after being employed by one carrier, checked out on line and then another offer from the other carrier. That is not to say that this will ever happen....
2. There are pros and cons to both QF and CX - and neither is a perfect job.....and nor will any job be perfect probably.
3. Should you enjoy a job offer with QF you could possibly gain: a great lifestyle in Australia, near to friends and family if they are from here, a mediocre wage, mediocre conditions, and a mediocre retirement. The days of being paid with high court judges is over. You will probably make some good friendships and enjoy your time at work. Health will probably be pretty good, access to top notch medical/dental is always there. Food choice if living in Sydney will be abundant and living in Sydney will probably only give you a 3 bedroom flat for your kids to grow up in. Promotion will be very slow. Will require careful investments outside of your job for a full retirement....But with the current of company operating style in Australia - you may be made redundant before retirment. Most importantly you can find a very very good coffee in Sydney.
4. Should you enjoy a job offer with CX you could possibly gain: a great and always exciting lifestyle in Hong Kong or one of their basings in Australia/Vancouver/LA/London/Paris etc. You will be away from family and friends, but with better access to staff travel, you will be able to visit them regularly. If you are married, you will not survive the job unless the other half is enthusiastic about Hong Kong. Conditions are good, wage is good, and retirement will be good if you retire outside of Hong Kong. You will definately make good friends (or you will not survive), access to good medical and dental is there (as part of the company conditions). Food choice is good, especially if you enjoy real chinese food (congee etc.) however western style restaurants are expensive. Outside interests from aviation are a pre-requisite for enjoying time off - and a desire for travel is well catered for with destinations such as Thailand/Malaysia/Japan only just a couple of hours away. Accomodation is subsidised or assiatance for mortgage, and gets better when an FO. Generally a small 2 bedroom 800sq' is covered by the SO assistance. Promation will be rapid. Flying is good - however I do understand that recently the rostering has been very ordinary. Rumours of take-overs by the mainland chinese have been flowing. So may make it to retirment or may not. Bring your own really good coffee into Hong Kong when on trips.
5. Being placed in this situation is difficult. However, no matter what the choice is, you must take the good with the bad, and give it your best. If you make the wrong choice....no problem - that is part of life. Keep moving forward and enjoying life.
6. Air travel is a global market. That means that there are jobs all around the world. All you need is determination, and knowing what you want out of life. Then find it anywhere in the world. Australia is a very small place in comparison to the rest of the world...and very niave in terms of job conditions... Should you not be successful with either CX or QF keep going - there is a big job out there somewhere that will suit you.
7. Good luck - and enjoy whatever comes. It is all about attitude.