Due to the wind pattern, the YVR->HKG flight is almost always more than 2 hours longer than HKG->YVR. A difference of 2.5 hours is not uncommon. The difference is usually bigger in winter months when the jetstream over northern Pacific tend to be stronger. You will be on the flight for about 14 hours for the YVR->HKG leg.
See
http://flightaware.com/live/flight/ACA7 vs.
http://flightaware.com/live/flight/ACA8
This should give you a reason to choose the opposite from what other people recommend here.