I've been flying Delta and Southwest a lot lately, and since they are rather different airlines, I thought I would jot things down.
In flight service:
I used to admire the Southwest service model, forsaking the bev cart and taking orders and delivering drinks via a tray. I have grown tired of this, though, because with a bev cart I can get 2 drinks at once, a soft drink and either coffee or water. Also Southwest delivers 1 snack choice almost immediately (graciously distributed out of a cardboard box or plastic bag) and the drinks come 20 minutes later as a rule, seems a poor plan. Usually the snack is graham cracker cookies which are not my fave. Delta always has 4 snack choices and gives out 2 bags, the same or different as you like. And the drinks and snacks come at the same time. Delta FAs also keep on their feet, doing a water run and a coffee run down the aisle every 15-20 minutes after the bev cart service. Southwest flight attendants do the one service, then sit in the jump seats and gab and gab. Only ringing for service gets more coffee or water after the initial drink service.
Seating/loading/bags:
Southwest wins on legroom, a inch more than the big 3 (incl Delta). at least on their 737-900. Greatly appreciated. Southwest allows free checked bags. Southwest also has the no reserved seating policy, with the boarding order decided at check in. My last four SW flights were for work (Anytime Fares) so I got boarding position A15-A30, which makes things easy. Last fall traveling on points I checked in at T24 and got C46, bummer. The huge advantage of the Southwest boarding system is that flights normally load faster. However my last 4 SW flights, even with the free checked bags policy in effect, there were still many people carrying on large bags and loading them up overhead, and so the time difference loading the planes between Delta (who charge for bags and have assigned seats) wasn't as great. Maybe 5-8 minutes faster loading SW planes than Delta, instead of 8-12 minutes faster previously. Every Delta 6-seat across jet has video screens at every seat and a USB port. SW seats have neither.
Other differences:
Delta has a conventional structure of different classes of seats, elite-based seating order, and a lounge system for elite passengers. Southwest has one class of service and no lounges.
Various anecdotes:
Only 1 delayed flight. On Southwest leaving Chicago last fall, and last flight of the day (after 8 pm), a high school tour group was coming back from DC and on that plane. One of the students lost a backpack, and they held up departure 35 MINUTES while they allowed the student to roam the terminal looking for the lost bag, there was even discussion about the student staying behind alone (!). Finally it was decided that the student would leave a phone number and we would go. I still don't get why it took 35 minutes for the SW staff to reach this obvious conclusion, the rest of the plane was there 34 minutes previously.
As to the FA joking on SW, some people like it but many don't. Delta FAs do not joke around. I prefer no jokes.