Some excerpts from We have it great on Southwest Airlines at flyertalk
Change fees. None at Southwest. Just raised to $150 at United.
Ticketless Travel Funds: Southwest invented it. United rarely needs it, the change fees cause almost no remaining value. You can’t look it up on the Internet, but keep your ticket number (the long one) and you can access it on the phone with an agent.
Call Centers: Southwest – based in the USA. United – based in India.
One-way bookings. Southwest: No extra charge.
Seating: Southwest: One seat (12F) with the most legroom of any coach seat on any US (maybe worldwide) carrier. The legroom may even exceed International Business class on legacy carriers (if you have never experienced this, please do sometime!) One more seat (11E) with amazing shoulder room as there is no 11F. Three more seats (11A-B-C) with more legroom than United’s Economy Plus.
Non-stop service: Southwest: Many point-to-point non-stops. United: I hope one end or the other of your A-to-B is one of their hubs. Otherwise, you are going to be visiting one of their hubs. Add 2 hours to your travel time and hope for no weather delays at the hub that day.