I'll give a contrary viewpoint here, just for balance. I am a Delta frequent flyer and will usually choose Delta when we have a choice of airline. That being said, they are an old legacy carrier (old style airline) with everything that goes with that; IMO they are one fo the better legacy carriers, but a legacy carrier all the same.
As for finances, I am a finance wonk by profession and training. IMO, you are going to have a higher likelihood of MidWest closing up shop than Delta. MidWest has addressed the shorter term issues confronting it, but does not really have the critical mass to be a guaranteed survivor; in other words, if MidWest were to go bankrupt, there would be a pretty decent chance that they would stop flying. Delta on the other hand is enormous, and since they have edged back from the brink of bankruptcy, they are on a pretty decent ongoing financial footing (not as strong as Continental, but stronger than USAir, United, American and Northwest).
So if you are buying in the distant future, like six months out, perhaps you might tend toward Delta for the security of knowing for sure that you will be able to take the flight. If you are buying nearer term, like less than three months, you might want to go with the greater convenience and comfort of Midwest.
Good lyck with your planning.