Delta and AirTran are the two airlines I fly most frequently, and for a short hop like DAY-ATL AirTran is your best bet.

No RJs, friendly employees, and the business class upgrade for $49 is a great deal, available to anyone on the day of departure as long as the seat is unsold. AirTran in coach is bearable---I'm 6'2" and it's cramped, but not any worse than coach on a Delta RJ and only a couple of inches less pitch than coach on a DL MD88. DAY-ATL is just an hour and a half block time, so if you're worried about legroom just get an aisle seat or ask for an exit row when you check in.

AirTran's frequent flyer program is based on credits, not miles, so you will earn free domestic tickets faster than on DL if your travel is mostly short hops. Four one-way trips get you a free upgrade; 8 one-ways give you a free one-way coach ticket. The biggest drawbacks to the FL program are that there's no option for international business or first class awards, and the program doesn't provide much bang for the buck on longer flights with the credits system.

I think AirTran is the most underrated airline out there...on short flights you get a better product (especially in business class) than you would on the legacy carriers, and the A+ Rewards program is decent.