Let me put it this way, if you're flying INTO china there is no doubt you would take CA over UA.

I only use miles and honestly I would have a hard time choosing CA Business over UA GlobalFirst on a number of factors.
It costs me the same, 80k miles each way when both are on Saver

If you goto Beijing as a transfer (both flights in business class), they will offer a free escort to the lounge or gate based on your choice/time.

Also at the lounges, you must be a CA customer in order to use the Shuttle service and the last time I flew CA (few days ago) on F, I had gate 36, which is a 10-15 minute walk. The little car transport got there in a few minutes.

I think in your situation you have a few things to consider
~CA FA's just don't speak any english at all, but win by a landslide in Service.
~Schedule and convenience of your location to your destination (which UA seems better)
~Can both access CA's lounge in Beijing
~No Escort/Transport in Beijing
~Air China uses new 77W's equipped with newer tech though UA refurbed there cabins.

Recently Air China changed baggage allowance to 1/PAX but since you are in Business you still have 2 @ 70lb.

I honestly haven't flown in UA international for a while, I'm not sure if there new flights have the new ambient lighting. I find it annoying when the lights turn on and off completely. Air china's changes colors throughout the flight and won't blow your eyes when you wake up.

Another side note is CA offer's flights from PEK to New York, maybe you can find a seat pop up between now and than?

I would take the CA route if I were you.