Suppose your inbound flight arrives 30 min late. Do you have any checked bags? With good legs you can make your connection to any gate in 15 min. before they close boarding. Don't bet your bag will. Your 5:30 arrives early at ATL, the 6:30 arrives more in the morning rush hour.

How critical is your trip? Just like I try to never book the last flight of the day, I also try to leave another conection between origin and connection. (the following applies more to feeder airports not hubs like DTW, but. . ) Suppose your 5:30 has a mechanical. There is a chance to waitlist and make it on the 6:30 and still make your connection. This is where it is good to have carry on bags only. Your checked bag will likely be stuck on the NoGo 5:30 plane and arrive in a few days.

It may be good weather where you are and ATL. But. The 5:30 is likely the first flight of the day. That plane arrived last night(even if it was 3 hours late) and has parked at the gate. The plane and crew are in your city. It will depart, and almost always on time. The 6:30 plane is probably a hop from another city. It may get a weather or fog delay from it's origin to it's first stop to pick you up. If you have to be there, take the first flight of the day.

add: P.S. DL runs one flight a day ATL-STX, your 9:49 AM. Miss it, and you are 1. likely to be stuck in ATL for 24 hr. at your expense, or 2. do a horrible multi carrier reroute thru MCO or MIA or NYC that might get you there same day but fuggetabout your checked bags.

Enjoy STX. Catch the free ferry from Christiansted to the Hotel on the Cay, grab a drink at the beach bar, and enjoy the view. Take your camera, it's THE postcard view you went there for. ^