Personally, I much prefer Southwest. We don't fly frequently, but I normally choose SW if possible, mainly because of the no checked baggage fees. My 2 DDs and I went to WDW in December without my DH...he had to work

It was the first time we travelled without him (two of them, one of me), and also the first time on AirTran.
We flew out of PHL. Trip down was easy and uneventful. Trip back was an absolute nightmare. First, looooong lines for security (about 45 min when we left out of MCO). I don't blame AirTran for that one, but it was the start of the trouble. We got our seat assignments and they had us all sitting in separate rows (my DDs were 9 and 5, so yes that was a problem!). I told them that when I purchased the tix, I chose specific seats. They basically shrugged and told me I was mistaken and there was no way to choose seats when you purchased tix UNLESS you chose the preferred seats and paid the fee. Well, I happened to be traveling back from Orlando with my sister, BIL, Niece, Nephew, BIL's mom, and friends of theirs--a family of 7. They ALL chose their seats and none of them paid any extra fees. When I told the gate agent about my sister being on teh flight with seats all together, all I got was "Well, it's a full flight, and I can't move anyone who has an assigned seat. But you can board once everyone else is already on the plane and ask people next to you to change seats with you." Really?? WHO is going to want to exchange their aisle or window seat for one of the middle seats that you assigned the three of us? And I assume there will be no room left in the overhead bins for my carry-on bags, which means you'll probably want to charge me the checked bag fee? I again reminded the gate agent of the ages of my children. He finally agreed to upgrade someone sitting next to me, so I could get 2 seats together. (Hello gate agent?? Did I mention that I have TWO children under age 10???) In the end my BIL sat by himself (which I knew he would), and one of my DDs sat with my Sis. But who knows what would have happened if I didn't know other people on the flight?
So, all the seat drama is settled, and we arrive in PHL at about 9:00 on a Thursday night. Of course, we were all sitting in the back of the plane (me, my sis, her friends)....So we waited for everyone to get off so we wouldn't need to fight the crowd. I went to go get my bags, and wouldn't you know? someone else had walked off with one of them!! I immediately talked to the flight attendants and a gate agent. They basically told me "Oh, yeah, that happens all the time. If they left THEIR bag, make sure YOU take that one. Otherwise you'll probably never see yours again. Good luck with that." HUH? So, I quickly opened and searched the other bag and got a phone #. Was it a cell phone # for the person? Of course not!! Had to leave a message at their home. Luckily, the woman's son heard the message as I was leaving it, called his dad i a panic, who called the woman, who called me on my cell, while we were both still at the airport. We made a quick exchange of bags. It was an honest mistake on the other passenger's part, but AirTran people did NOTHING to help.
So, my advice, pay a bit extra for the preferred seating if you really want it and don't carry on a black wheeled suitcase!