I've been here at DAL since 2000, furloughed for 3. I've seen the company at its best, its worst, and right now we're somewhere in the middle. I truly believe there is a bright future for our new hires. BUT, a good friend of mine from the regionals is now a Captain at SWA...hired in 1999.
There is no doubt there is a ton of stability at SWA, but nobody can predict the future. You've done your own seniority analysis and I think that's a pretty good excercise in forecasting your future lifestyle. I will retire (God willing) at just under 30 on the seniority list here. If we never buy another jet, I'll be a 777 captain for 8-9 years, and an 767 captain for 10 before that. Sounds similar to where you are at. Heck, even if you decide INTL is not for you, you'll decide that at a young age and can hang out as a super-senior narrowbody captain.
Right now upgrade here is 10 years (NYC MD88). IF we get the mystical growth airplanes we keep hearing about, that will come down a bit. If it doesn't, upgrade will increase to probably 15 years. But if you have a 30+ year career ahead of you, who cares...after a while you'll still be making over 100k, flying your choice of planes, bases, and having a nice life.
I'm junior and really enjoy my life here at Delta. People are fun to fly with, the planes and mx. are great, the operation is generally well-run, etc. BUT, why not take the SWA interview and check them out too... at least when you finally make a decision you can feel like you left no stone uncovered.
Good luck!