I was tempted by CO, and even status matched to them, but after flying a mix of CO & AA I realized AA was better for my cheapskate patterns. I enjoy upgrades and the CO method of (a) prioritizing upgrades based on fare paid and (b) almost all upgrades waiting till the gate turned me off. AA's upgrade process works great for me, even when I was a PLAT I thought it worked well. If my travel were more business travel with a higher fare class then maybe UA/CO would work better for me. But, as it is, AA works great for upgrades. And I enjoy the eVIP benefits as well without having to worry about buying a more expensive fare class just to get a chance at an INTL upgrade.

For service, it was very mixed on my CO flights. On the good ones it was better than AA, but it was really spotty. I'd say <20% of my flights were with crew better than AA. Most were about equal and a small # were worse that my typical AA experience.

Not much info on Delta. Only flown 4 flights on them in the past couple years. Biggest concern there is ability to redeem miles at a decent level. I like AA's options for Australia (getting lucky and being flexible though), Asia (CX was easy), and Europe (though I avoid BA on transatlantic segment). Been pretty easy for me to find Saaver awards for all my reward travel with being a bit flexible on dates.