Airtran is a very reliable / reasonable low cost carrier. Smaller than Southwest they are a national carrier that has its major hub in Atlanta and competes very well with Delta.
They also compete very well with American in Dallas. I wouldnt hesitate to use them and I work for AA.
Flying from Orlando to DFW you will have a lot more choices on times from AA. There are 11 direct flights a day. Airtran I think has only 3. You can connect through Atlanta though.
You can also take Delta to connect through Atlanta as well as some of the other Major Carriers and connect through their hubs.