Delta customer service is much better. AA is extremely rude.
Delta's on time percentage is better.
Delta is much more accommodating when they delay or cancel a flight. I just got my flight to Dallas cancelled by AA and their response was "too bad, we can get you there two days later." They didn't even call to cancel, I just got an alert.
Delta bends over backwards.

https://millionmilesecrets.com/guides/american-airlines-vs-delta/#:~:text=Delta%20is%20(and%20always%20has,quality%20and%20on%2Dtime%20percentages.