I have flown on both United and Air New Zealand and I would vote ANZ. I just did the trip to Sydney in First Class and although the sleeper seats were great, the food was not so good and the service seemed cut down since the bankruptcy. On the otherhand last time I went ANZ, I was amazed but the service, food, wines and 747. Uniteds 747 seemed a bit worn.

Of course you have to consider connections. Its my impression if you go via ANZ you fly IAD - west coast - AUK - SYD. On United it's IAD - west coast - SYD

According to the Star Alliance websight the fastest route to SYD from IAD is thru LAX, United flights 177, connecting to 839. That takes 21h 24m. The next best is thru SFO and its flights 951 and 863 at 22h 55m.

On return flights thru LAX or SFO are 20h 05m and 21h 12m. Actually I am not even sure if ANZ is an option for you unless you want to spend all your tume in the sky.

One trick I did on my way to SYD last month was I flew to SFO and arrived early in the morning, rented a car and went to San Francisco for lunch and then got a cheap hotel on Priceline and used it to rest and freshen up before the evening flight. I bot the car and hotel for under $50 and it was well worth it. You can do this at either LAX or SFO.