American flies from Chicago.

United flies from Chicago and San Francisco, which is great in cases of irregular ops, becuase you've got a fallback and more capacity in the system to handle it.

United also flies to Beijing and Hong Kong from its hubs, including 3x daily to Beijing. That's essentially three more choices to get to China, including Shanghai in the case of irregular operations -- a hop from PEK to PVG or SHA is really easy.

United is rolling out its new premium product in its 747s, so if you're especially concerned about the next year+, you're going to get that product more and more often. I think the broad consensus is that United's new premium product is better.

United offers 747 service. You might like that better than 777 service, since there's an upper deck for business class travel. (You didn't specify what class of service you're planning to travel in.)

Honestly, for frequent transpacific service to Asia, especially China, you really have to choose United over AA. I suppose there will be some arguments to the contrary, but you're way, way less likely to end up stranded on either end in the case of operational problems with UA simply because of their capacity and frequencies to major cities there.