The Qantas partnership ends on 2/28, so QF is not an option if you want the miles.

Another not-quite-obvious possibility is Air New Zealand. You would not be flying direct, but rather though Auckland. However, if this is your first really long flight, that might be an advantage because LAX-AKL is 2 hours shorter than LAX-SYD. Also, since you're *G, you can relax, snack, and/or clean up in the NZ lounge at Auckland (with free WiFi) then take a 3.5 hour flight to SYD.

NZ's economy seats are 17.8" wide with a pitch of 34" and each seat has good IFE with 8.5" screens. UA's economy is 17"/31" with E+ at 17"/34-36". NZ's mid-grade service is called Premium Economy. Premium Econ has a coach-ish seat, but it is wider (18.5", has more legroom (38-40") and a (manual) leg rest. Almost all of PE is upstairs on the 747; it's a nice, small, quite cabin. Also, PE now has the same food and beverage service as the top-grade Business Premier. The cost for PE is about double that of coach.

On a recent trip to SYD and MCY, we flew NZ econ westward and upgraded to Premium Economy on the way back. This combination worked well for us and the effective "price boost" was only 25% per ticket.

I'm sorry to sound like an ad for NZ, but I've been really happy with their seats, service, food, and lounges (and it has nothing to do with the first-class upgrade that they gave me from LAX-BNE for my first trans-pacific flight ;-).

NOTE: In September, because of daylight saving time, you will not be able to connect to any US LAX-PHL flight from any NZ AKL-LAX flight because, except for the US redeye. United has a LAX-PHL flight that is late enough to make that connection.