Does one have better flight times? Is one non-stop? That is what I would base my decision on. For us, Airtran has a lovely non-stop flight to MCO in the morning and wonderful non-stop flight home in the evening. (from FNT)
We have flown both and they are both fine. Both can move flight times - some will say Airtran moves their flight times more than other carrriers, but that isn't my case, Airtran moves thiers less that Detla for me.
Keep in mind you have to pay for seat assignments on Airtran ($6/seat).
Emily