Depends what your priorities are. United has Economy Plus and a friendlier frequent flyer program; Qantas has better in-flight amenities and a peerless Australian domestic route network. There are many other variables.
See
https://www.flyertalk.com/forum/showthread.php?t=375227
https://www.flyertalk.com/forum/showthread.php?t=503487
https://www.flyertalk.com/forum/showthread.php?t=321579
Remember too you can get to Australia on other airlines via connections in AKL, ICN, HKG, PPT and so on.
Last edited by choster; Dec 28, 2005 at 7:57 am
Reason: added some connecting points