I've heard good things about Eva's E+. Don't know how the fares compare between SEA & YVR. Eva departs SEA around 4a.m. and from YVR about 1a.m.
A co-worker recently flew YVR-BKK on Air China. The connection in PEK was about 8 hours in one direction, more reasonable the other way.
Transit visa not required for travel on Air China.