You could of course buy 2 one-way tickets. Do one leg in J and one in F and then you could be the judge whether it is worth it or not.
The other advantage is that if you have to amend just one leg of your booking you can do so without having to cancel the whole booking and start again.
Received wisdom is to do the day time (if there is one) sector in F and the overnight in J. That way you can maximise the Food and Beverage amenities in F, whilst still getting a good sleep in J. For my money, I'd do the outbound in J and come back in F; the lounges in HKG will be nicer and also you end your journey on a high. The new CX J class looks very good though.