One advantage with QF over United is that you can do the stopover or open jaw while in Australia. Since UA only flies to SYD and Melbourne, you wouldn't have that option. I considered UA using US miles, but then I would not be able to get my SYD-CNS-SYD for free. Remember the QF award is more expensive in terms of US miles than the UA award.

If you decide to pay or use miles for a UA award, your US status will get you E+ seating. That'll make the journey much more comfortable if riding in coach.