If AC is routing through YVR, definitely choose CX. As AC flies her 10 abreast 77W on HKG-YVR.
I would not be too optimistic regarding upgrade chances. Both airline's HKG-YYZ J class seem to be full for the past couple times I flew with them. Nonetheless, both airline's hard product is very good. (of course, CX is superior for soft product)
All my options are direct flights...YVR timing sucks and you arrive YYZ in the wee hours of morning.
I wonder if the Y hard product difference between CX and AC justifies the extra cash?