Whats the preference MEL-SYD-YVR or MEL - AKL- YVR?

Based purely on transit experience, I'd choose via AKL every day of the week. 

The only problem I have with AKL is their rather long Customs check, only two lines to do all the work while in Melbourne you got five. Not related to the question but just my opinion on AKL.

If you are only transitting AKL, Customs is not a problem.  Coming in on an int flight, and going out on another.

If arriving, and staying in NZ or visiting, yes, it can be long, esp if picked for "special treatment".