I suggest looking at the routes that you will fly most and assessing which airlines offer the best flight options. Upgrades are important as are the other perqs - but - when you start flying all the time, the most important thing is sometimes just very simple - which airline has a nonstop etc.

Being based at ORD is excellent. You have both UA and the Star fleet of airlines and AA and team One World. DL is unlikely to be your pick unless Skyteam partners fly nonstop to ORD from locations that matter to you (as some may). .

Were I in your shoes, I would think about what you like and dislike about AA and then see if the other option(s) offer a better alternative. If they don't, then I'd probably stick with the devil you know.