Vegas Gambler hit the nail on the head. They're all pretty close now, but DL used to have a pretty big lead in this area for domestic first IMO.
I know your only asking about AA/DL but will rank the 4 in my experience (and will only reference this year flights as they are all getting slightly better or worse then they have historically been... To add context, I fly Delta and Alaska the most along with 8 flights so for far this year on AA and 6 on UA.

#1- Delta- Highest PDB % , 2nd best food, worse in seat space overall (however I really like the A321 neo seats), above average FA's.
#2- Alaska-Almost no PDB, best food option, most seat space, but FA's are average to below average on the whole
#3 AA- Consistent PDB, decent food, decent seat space, pretty good FA's
#4- United- Consistent PDB, not decent food, average FA's, seats

The top 3 are pretty close overall, but safe to say they are all less then they were in years past (mainly before 2020)...

If you slide over into the Delta forums, you will probably see no love for Delta first right now, as the majority of the postings are upset about the recent announced changes to their FF program and Deltas continue overuse of the word Premium by their marketing and C-Suite since the "premium" experience has declined from the pre 2019 years, but in IMO still is better then the others overall.