Detroit City
City airport has only one runway suitable for large aircraft, 15 - 33 and it's 5715' x 100, if you use the "private" portion of the taxiway / runway. Both ends have ILS approaches down to 250' and 1 mile.
Southwest, when they flew there, only seviced IND and MDW nonstop, and ProAir didn't go much further, maybe ATL, even through they had direct service to Florida on the weekends. (fuel stop enroute)
I did hear on the radio not to long ago, Delta (i believe) was going to start service out of City again. I'm sure it was ComAir or ASA, going to CVG or ATL.
If you ever travel into City Airport, make sure you're "packing some heat" because I don't know anybody that wants to go to that part of town, in daylight, let alone after dark.
As far as Flint goes, a very large percentage of the population of Detroit lives in the northern suburbs making FNT an easy commute for millions of protential customers. The disadvantage to FNT might be the lack of airline traffic, or choices on one hand, but on the other hand, that lack of traffic means fewer delays, close-in, cheap parking and you don't have to put up with Northworst.