I fly both UA and AA out of ORD. My decision is usually made on who has the better fare & schedule for my destination. In coach, I don't see much difference between the products. My personal experience is that AA handles IROPS a bit better, but that could be just my experience based on a very small sample.
I think the rjque's observation about the lounges at ORD is spot on.
Lately I have been flying AA more and more for personal travel as my wife and I fly business class to Latin America and the Caribbean. AA is usually cheaper for this and I find the the domestic first service on AA is somewhat better than UA.