Having flown that route (and into MEL/BNE/CNS) for nearly 11 years (on United/Continental, Delta, Qantas, and now, VAustralia), I noted the main difference is, how quickly do you want to feel as though you've left the U.S.?

Qantas and VAustralia (just like Virgin Blue) are Aussie from the beginning, including better food, better services, and (generally) more talkative seatmates. Australians travel all the time (they are one of the few countries in the world where more residents holiday out of the country than within it), and they enjoy telling you about it, as well as the best parts of their home.

Delta and United are U.S. from the beginning, and (IMHO) tend to be filled with more travellers heading to Australia/NZ for holidays or non-business trips, even up front. This, to me, translates into more inquisitive discussions from seatmates, more BS PAs about duty-free crap, and worse food selections during the flight. It also means waaaaayyyy more gate lice and kettles getting in the way.

YMMV, but it you're headed for holiday, and you don't like the long-haul duration, you might consider UA/CO (blasphemy in this forum, I know) IAH-HNL-GUM-CNS. It breaks the trip up a bit, and you already are up in the top end when you arrive. You can always CNS-BNE/SYD/MEL after going to see the Reef. The total costs for that routing are usually less than heading to SYD or BNE or MEL directly, even after you account for the routing down south (and you could work a day stopover in Hawaii or Guam if you desire).