Prognosticating on whether American or United will be a better choice in ORD is a fools game. You can rest assured that the major questions that will affect your career will have nothing to do with ORD, American, or United. The problems will be cabotage, drones, a global depression, or some other equally separate issue far beyond your control.

Pick the company you like most. Live below your means, and THEN and only then will you be happy.