I would also vote for AA as the no-brainer. On top of the reasons mentioned above, if you don't get an E+ on united, you have to suffer for 6-7 hours.. On AA you have MRTC in the entire cabin.

Also, if you're doing this flight every 2 weeks (@14k butt in seat miles a month), you'll get EXP in about 7 months. That will get you 8 VIP upgrades that you can use on ANY pulblished fare to upgrade to business. If you end up doing any US travel, you'll get free domestic upgrades as well. You already have the lounge thing taken care of to start with your quantas status. So do the plat challenge, you'll be plat halfway through your second roundtrip for the 100% bonus.