Haven't flown the YVR-SYD route on AC, but have done a similarly long YVR-PEK on the same equipment and it was perfectly acceptable. Good personal IFE, free booze, pretty decent food and reasonable seats. If you were to have a straight shoot-out between NZ and UA, it has to be Air New Zealand every day of the week. UA is only worth it if there is a heavy discount. Usually the connecting flight from NZ would be an issue (just over 3 hours in a narrowbody) bu if you're already visiting Auckland anyway, then the issue is neutralised. If you want, you could splash out on an EK flight from AKL to Australia or vice versa. To/from SYD has an A380 service while MEL I'm pretty sure is still on 777s.

AC, NZ and UA are the only direct *A options to Australia (OK, NZ is "direct-ish") from North America. You could of course fly via Asia, but that is going to take a fair while longer and likely be more expensive too.