I've done SEA-YVR-SYD via AC and also SEA-SFO-SYD on UA. AC definitely gets my vote for better service and overall experience. The only thing that differentiates UA is E+ seating otherwise its an average offering.

As for transiting through YVR, it does add some extra time, especially on the return as you need to clear immigration/customs twice but as long as you leave sufficient time between connections, its not too bad and stressful. The turbo props are noisy and bumpy but thankfully it's a short flight and over soon.