Funny, at my interview at Flex this week there were Airtran drivers there, one I know of got the job, so did I.
On the road I meet many Ex - 121 drivers flying for NJA, and they are much happier on this side of the fence (but I do admit they all said they wanted to get back into corporate flying after realizing they made a mistake going to the 121 world)
The guy I currently fly corporate with was also a ex Airtran FO, he lasted 2 years before coming back to corporate flying.
121 vs 91k is a pure personal decision I think, based on what you like to fly, where you like to fly, who you like to fly, and of course when you want to fly.