Both UA [with E+] and AC with its 32-34 inch pitch offer the best legroom option versus TG and SQ, or even CX or other carriers via their respective gateways. No matter which choice you make, you'll have at least one connection to make. Flying AC will require a change in HKG or NRT to TG or NH/TG respectively. Also remember some cheaper fares on the Asian carriers do not garner Aeroplan miles, so watch out for that wrinkle. [Though if you are flying at the last minute, you should not encounter this problem.] You will likely find cheaper fares ex-SEA, even on AC via YVR and HKG, than if you started the trip at YVR. And given last-minute Ys may be more pricey when booked directly with the airline, you might consider looking at a consolidator ticket through a good TA, and could buy a J/C fare for an attractive price equivilent or less than a last-minute Y fare through the airlines.

If you do connect at HKG to TG, their lounge is very nice and offers fine dim sum.