Wow, I went to bed and woke up with such great feedback. One of the major considerations is arrival time, so that I can spend the whole weekend sightseeing in Beijing. This effectively ruled-out DL and AA (plus the European carriers). CX is out partly for the same reason, plus I couldn't justify on an expense account the price being triple all of the other carriers.

Based upon this, it seems like a choice between AC and CO. Both are equally convenient from Boston. I'm thinking of going with AC based on it seeming like a safer-bet based on the feedback (guaranteed flat seats, etc.). Never used their miles program, hopefully they will be useful in the future. Any issues with customs and checked baggage with the layover in Toronto? Hopefully 2 hours is enough time.