I'm not sure I'd agree that all four carriers are "great," but I agree that the differences aren't big enough to make that a reason to pick one over the others unless all everything else is nearly equal.

In your case, they're not. You're near one of their hubs, DL's at CVG, and not near a major hub for any of the others. Unless you enjoy the prospect of being forced into an extra connection any time you go nearly anywhere, that (for me) ought to settle it.

(I often make voluntary connections for extra miles, time permitting and fares being about equal, but that's not the same thing. I want it to be my choice, not a requirement.)