Depends on what is more important to you and which airline (plus its partners) has the best route map for you.

AA will provide a more pleasant Y experience from your first (non-elite) flight. AA also provides almost-humane customer service to its low-elites, something UA, DL, and US absolutely do not do. (Don't know about CO.) AA provides what I would call semi-friendly service to its mid-elites, which is unique in the industry. I'm sure top-level on both carriers garners good service.

AA will never get you unlimited upgrades unless you are buying full-Y. I've heard that CO wins higher marks in this area. I'm an AA flier and I've stopped obsessing about upgrades: domestic F is only about two inches and a couple free beers better than Y, so that isn't all that important to me.

It'll probably come down to whoever has the best schedule for your routes...