I would definately go for CX because of the entertainment and the seat. The seat in qantas is very tight! 31 pitch. I am 165cm tall and i find it hard to stand for 8 hrs. I have tried Qantas on HKG-SYD Vice Versa. Also in qantas, the entertainment screen is very small 5.5inch only!!! In cathay the monitor is 9 inch. and the entertainment options arent as good as Cathay Pacific. The shell seats are good aswell in cathay. Also, the food is not so good on Qantas. So i guess u really should go for cathay!

Hope this helps!