I have chosen AA as my carrier of choice. They have not always been the best, but they:

1. Service the domestic markets I need access to most frequently.
2. Their worldwide and domestic partners fill most of the other needs that I have.
3. Their frequent flyer program is fairly generous as far as upgrades and availability go.

I have noticed a lot of the same in regards to service levels with AA and UA. I have experienced the poor non-elite treatment from UA. I have experienced better treatment as a oneworld elite from Qantas and BA more often than on AA.

What it comes down to is that you are never going to have everything perfect and go your way when you are dealing with huge corporate machines like AA and UA. One will be better in one area, the other will be better at somthing else. Sure, you can vote with your dollars, but really, as an individual, you don't matter that much to them. As a large corporate account, maybe.