You ask UA or CO. However I am confused because you say AA? Which routes "specifically" on CO or UA? Transatlantic or domestic?

Anyway here are my 2 cents. Domestic CO I avoid the B737 as I find it terribly uncomfortable. Minimum leg room, narrow and hard seats. They say they are the only ones who serve food but I would prefer to buy it, based on what they give you. Transatlantic it was OK except for the food which was unedible. Comfortable seats on the B767 with 32" legroom.

I personally don't like CO and would opt for UA though I have not flown long distance with them since 10 years. I liked it at the time.