I am trying to fly only on DAL/Skyteam, for 3 reasons, and 2 of them are because of DAL mgmt decisions:
1-DAL has the most larger aircraft:
I flew Columbus to MIA on an Eagle 50 seater.
I flew ORD-MIA on a United Express Mesa 50 seater.
I looked at LAX-GRB:
AA and UAL both had 50 seaters from LAX-ORD-GRB.
DAL had an A-320 from LAX-MSP, then a Compass ERJ-175 from MSP-GRB. I booked the DAL itinerary.

2-DAL has a lot less cancellations than UAL/AA, and they don't fly through ORD, which is over-capacity during rush hour, so the slightest WX problems cause delays, which AA and UAL both blame on ATC!!!

About the only reason I am not flying DAL more, is that sometimes DAL/Skyteam doesn't go where I need to fly, or sometimes DAL has no seats. Occasionally AA/UAL has better schedules, but not often.

If it is not DAL, then I try to fly AA, and that is for two reasons:
1-AA has Qatar in it's network, so when I am flying thru or to/from the Middle East, I can get airline miles on AA (can't with Emirates, Etihad or DAL/UAL).
2-AA seems to have a better network, and less cancelled flights than UAL.

Overall, for American carriers, I really prefer DAL.

My buddy flies feed for both UAL and DAL. He said it is Night and Day when comparing the support he gets for his flights. The DAL personnel are MUCH more helpful, and the DAL operation runs much more smoothly. He said it is really obvious that DAL is doing a much better job than UAL.

My last point: I hope that DAL/AA/UAL can compete effectively vs. EK/EY/Qatar (and Turkish). They are really starting to move a lot of passengers (including me) to and from the US, from all points of the globe, and with a MUCH higher level or service, food, and passenger amenities.