I fly UA twice a year via EWR, SFO or ORD to PEK or PVG and back. Usually a 777 but also 747 (via SFO). Think the number of seats across are different on the 777 depending on if there is a GF or not.

Never been on AA although my colleague who is a AA fan does. I agree with what other posters have said, UA flights are full and the service is less than "friendly" although I've had great service via SFO and EWR to be fair.

All of UA's BF seats are lie flat. I don't know if that is true for AA. Although UA's seat isn't wide, its comfortable enough. I've also flown CX which is much better but the extra hours in the air/connection are tough to justify unless your final destination isn't PEK or PVG.

One other consideration, I haven't checked but the UA flight got into China at an early afternoon, I remember my colleague saying AA gets in very late so connecting is impossible.

The reality is that I just want to sleep as much as possible when getting on the flight. On the lie flat seats, get an aisle or you will need to climb over your seatmate to get to the rest room.