Delta. I like having the choice of switching planes or flying international if I want. If you decide that international isn't for you, you can stay domestic and enjoy the seniority. I haven't hit 3rd year pay yet and I can hold narrow body captain or wide body F.O. in NYC. I suspect I may be able to hold wide body in my current base by the end of the year...probably narrow body captain at year 3-4.
If you want to live in a SWA base and don't want to ever have to commute then SWA is probably the best bet. I know lots of guys who have left one airline for another, for home basing options. Just keep in mind that Delta used to have bases in ORD, DFW, MCO, PDX...you get the point. There were two former SWA guys in my DAL class and a 5 year SWA guy in the class behind me. Then again, a buddy had class dates for both and stuck with SWA since they gave him an earlier class date...having the choice is great!
Having flown the 737 for a few years now, I can't imagine having to fly that thing the rest of my life. I don't even care about the archaic overhead panel, it's just an uncomfortable cockpit, unless you're 5'5''. As to the 88/717/737 comfort and design...been on the JS of them all, the 717 wins hand down! The 320 blows them all out of the water.
Both are great companies, best of luck!