Mind you, this is not apples to apples:

UA: LAX-EWR PS
DL: LAX-DCA in Econ Comfort
AA: IAD-LAX in F

I just want to share a few observations: Note, I am Plat on UA and DL and mid level on AA (comped)

1) Upgrades.
UA is rarely possible to secure an upgrade on using a regional and nearly impossible to get otherwise, unless it’s a buy up.
Didn’t try for a DL upgrade.
I put in for uprgrades on AA with 5 500 milers and was confirmed two days out.

2) Check in and airport experience.
I have UA check in down to a science. I rarely check bags. UA at LAX is great. Love the lounge. Security is a breeze (with pre-check). Got from front door to club within 7 minutes.

DL check is was extraordinary. I was traveling with a group of 11 teens. The moment we walked into the terminal, an agent spotted us and escorted us to check in. I was able to check three bags under my status, and my DL AMEX allowed me 8 additional bags. Short security line. Lounge was crowded, but nice. I know it will be redone soon.

AA check in was fine as well. I was again with a group of 11 teens. I got two free bags, and we had to pay $25 for each of the others. Security was longer, but fairly quick. No lounge at IAD.

3) Flight
What can I say.... I really like UA. The food is fine, the entertainment is comprehensive, The PS seat is great. The FA’s tend to go above and beyond.

DL. Again, in their version of Econ Plus. Good legroom. We got a free small meal as well as ice cream before landing. Entertainment was fine.

AA: Regular domestic F. I was able to pre-order a veggie meal, which was fine. Found the FA’s friendly, but perfunctory. Entertainment also comrehensive.

I think when it comes down to it, I prefer F in UA more than AA (even though the AA upgrade - based on one experience - was easy. Mind you, I prefer regular F on UA better than regular F on AA. All the more so with PS on UA.

I prefer the phone experience with DL most. I find AA’s pretty poor. (I never seem to know the secret handshake to get to a human!)

With status on all three (as well as with Alaska), it’s nice to be a free agent. All that said, at the end of the day, I still find UA to be my preferred carrier!