First off, how badly do you need the miles? I'm assuming you're not worried about maintaining status if you're willing to give up 50%. It's a fair bit of mileage, but...

UA 747-metal is not going to be pleasant. That's a long way to go. If you have your own entertainment, however, that you can keep charged for a decent amount of time, it might not be so bad. I wouldn't personally give up 50% of the miles for things like food. However, ACs 777-300ER (lower density, as on this route) is by far the most comfortable in coach I've ever flown.

I wouldn't fly Qantas over AC, simply because of the aircraft type and though connections aren't a treat, lounge access helps.

If it were me, all things considered, I'd go with AC.