I have had small and dated planes in Delta flights, with limited room for carryon baggage so you may be forced to check your bag before boarding. Maybe it depends on both your departure and destination? I now avoid all of Delta’s “partner” flights.
Def. true in terms of size.
Take it for what you will but according to wiki for Delta:
"one of the oldest fleets of any American airline, with an average fleet age of 16.2 years as of September 30, 2018. Its oldest aircraft types are the MD-88, MD-90, Boeing 757-200, Boeing 767-300, Boeing 767-300ER, and Airbus A320-200, which have an average age of 28, 21.6, 21.1, 25.3, 22.3, and 23.1 years, respectively. Its youngest aircraft types are the A321-200, A350-900 and Boeing 737-900ER, which have an average age of 1.0, 0.7 and 2.7 years, respectively."
Also from wiki: "As of February 16, 2019, Delta's fleet consists of 887 Airbus, Boeing, and McDonnell Douglas aircraft. Delta's 520 Boeing aircraft make up about 58% of its fleet, while its 248 Airbus aircraft make about 27% and 119 McDonnell Douglas aircraft make up about 15%. Delta's 734 narrow-body aircraft comprise about 83% of its fleet, while its 153 wide-body aircraft comprise the remaining 17%."
It's just like with SWA--it's a toss up for when you get the older planes and the newer planes. I do believe with Delta you can check the type of aircraft you'll be on but of course that can change if there is a need to change the aircraft.
I will admit I did like the Delta planes with charging ports for each seat and the tvs so I'll give props to Delta for that.