Just remember the extra charges on AirTran. It's $15 for the FIRST bag per person each way, $25 for the SECOND plus an additional $6 per person each way ($20 for exit rows) for reserving your seats in advance. Typically you feel forced to book your seats if you're traveling with young children or you don't want to risk not sitting with the other members of your group. There's also a $75 Change Fee if you change your reservation.
In comparison, Southwest has no baggage fees for the first and second bags and you don't book your seats in advance, you just have to check in online exactly 24 hours in advance to get a spot towards the front of the line to get on the plane. You then can sit anywhere you like and usually with your family if you're towards the front of the line. There's no Change Fee if you need to switch your flights.
It's so tough trying to compare apples to apples nowadays.
Cheers!