I would say most of land below St. Louis and Kansas City is Southern. And St. Louis, Kansas City, and the North is Midwestern. The Northern parts of the state are too moderate to be southern, considering how incredibly white they are, like Iowa. The thing I see that always bugs me is that in Northern Missouri most counties went >60% Romney, yet right when you go into Iowa you're immediately into swing/toss-up land. Kind of like the Tennessee/North Carolina thing.
Iowa and Missouri are very different. I'm originally from Southern Iowa, where people speak with a flat accent, but when I was in Northern Missouri, people definitely had a twang.
I do consider Missouri to be a Southern state, culturally (aside from St. Louis and maybe Kansas City).