AC uses Embraer 175 regional jet for this route. Don't know what business class is like on this plane but I'd rather not spend the 3+ hours in a regional jet if doing this route monthly.
AA uses Airbus 319 jet.
AA will give you options on routing via ORD in case of issues with flight, weather, etc.
AC you will have to ask them to put you on UA metal.
I'd go w/ AA if their timings work for your needs.