If you choose Endeavor and pick DTW. Spirit has 2 flights a day from ATL-DTW. IMO, for Spirit, it's easier than trying to get on a the Jumpseat of a Delta or Southwest flight. You can list online and see if there's anyone else listed. Once you list and check in 24 hrs out. The JS is yours, as long as no Spirit Pilot is listed, but if there was you will see them listed (Most of us will not list or show up at the last minute). With Delta and Southwest you don't know who is listed and if they have checked in until you get to the gate. We take many UAL pilots and FA between MSP-ORD for this exact reason.