We usually fly US Airways, but we've also started to fly AirTran. We did fly Southwest, and I have to say, I'm not really a fan of them. Out of your choices, I'd go with Delta. We've had good experiences with them.
1.) full credit for cancelations, changes, or price drops. Can't tell you how many times I have done this. If I had to pay a fee each time it would drive me crazy, I am spoiled by SW.
2.) great sales!
3.) nonstop flights to Orlando from my local airport