You do have to watch Spirit and their charges, they're a bit more aggressive than anyone else. That said, they treated me like royalty the one time I had to use them to get to Aruba. I was traveling on my jumpseat pass and the experience was fantastic. A few people complained because I had a larger carry-on than their rules permitted but the flight was fantastic and the "big front seat" is priced appropriately for what you get. Same service, just a bigger seat.
DL does have pretty good competition in ATL. AirTran is running some fantastic specials right now. They just haven't expended into the islands, yet. For $50 a head more than my standby passes, a friend booked a christmas trip to Orlando for his family on AirTran and the flight is direct, no connections. Normally direct flights cost a bit more as people are willing to pay a bit more to avoid a connection. DL will usually be the most expensive for you no matter what.