I fly both....it all depends on price and schedule. I prefer JB, but I have no issue with SW. I like the private entertainment system with JB, but the free checked bag is a plus with SW. Now that JB is charging for all checked bags, the playing field is similar.
I just booked SW for a trip to WDW in two weeks. While the fares were only about $5 different, I went with SW, out of Providence, because of the flight times.
My issue with JB? Sure you get assigned seats. But.....it's getting so that aisle seats are more expensive, there are rows that constituted extra leg room seats. Exit row seats cost more. Now, they charge for that first bag. No, you can't pre-choose your seats with SW, but in all reality, if you get a boarding slot anywhere before B25, you should have no issue finding acceptable seats...for your entire group, unless it's a group of 9!!!