We had the same 'issue' recently and opted for the QF flight because it left at a slightly more civilized time (11.59 pm versus 1.20 am for the CX flight, iirc) and also gave us an extra hour on the ground in HKG to enjoy the Wing and have a shower and a leisurely breakfast.

For sleeping, I prefer the QF Skybed to the (old) CX seats, which would just reinforce the decision to take the QF flight.