What do I know, I work for DAL.
First, I'd pick the one closest to where I want to live.
If that isn't an issue, I would pick SWA.
My reason is the type of flying.
I've done both exotic long-haul international and flown the Shuttle. I much preferred flying the Shuttle. After all what's fun for a pilot, take offs and landings. You get a lot of those at SWA.
You can keep the back side of the clock struggle to stay awake. Puking your guts up after eating who knows what in some god forsaken backwater sucks. I'll take controllers who speak English and radios that work any day.
But, that's just me. You decide what works for you. In the end, you go with the one that offers the job. You won't know if you made the right decision for another 30 years give or take.
And, changing airplanes is highly over rated.