So I am one guy who left United last year for AA. Why? Retirements greater than any other mainline and domiciles. Yes many things were better at United but they also came out of bankruptcy longer ago so they have had more time to fix things. AA isn't bad. I have met very few bitter pilots, most are all great people and enjoyable to work with.
True we lag behind in pay and work rules but it is not horrible, yes it can improve though. If you are 50 or older and had the pick of whichever airline I would say go UA or DL because their contract is better now and ours won't get renegotiated until 2019. However, if you are younger than 50 I'd say go with Domicile first, then AA if they are all equal after that. Reason I say that is because the movement you will see at AA. Delta has hired twice as many pilots as AA and I'd say United has hired 1.5 as many. No matter what you do you can never get those numbers back. That said we also have more pilots that will retire % wise and total numbers compared to DL or UA over the next 10 years so upgrades and job security will be better. Also majority of the pilots hired at AA over the last 2-3 years are older on avg by far compared to DL or UA. So those hired in front of you at AA will retire sooner vs later in your career.

Contracts and CEO's come and go just like Presidents. If you have 20+ years until you are 65 then pick by Domicile and QOL first and then contract. All the big 3 are great if you live in Domicile IMO.
None of us will ever know if we made the right decision until we retire. My dad retired this month from Delta as a 777 CA who had a 31 year career with them with the last 10 as a 777 CA. Multiple times in his career if you asked him if Delta was the best airline when he got hired or at that time he would have said no. Ask him now and he says yes for sure. In his words,"every airline will spend time at the top and at the bottom over a 20+ year career...." He has seen it and so have I growing up in an airline family. So I'm happy with my choice of AA and I guess in 2043 I'll reassess and see if it was the right choice.