In the plains states, the eastern portions are 100% midwestern, while the western portions are about a 50% midwest, 50% western split. Even areas in central Nebraska are more closely aligned with the midwest.
The exception is Oklahoma which is sorta of an even blend between southern, western, and midwestern.
As for most bohemian, it depends. People mentioned Madison and Ann Arbor, but as a UW grad, I'd say Madison is probably the least artsy/bohemian of the 11 cities I've lived in, right up there with Washington D.C. as a well-educated, yuppie dominated place
Some of the larger cities in the western part of the midwest have a bit more of that going on - K.C., Omaha, and of course the twin cities. In general, river cities generally seem to have a more bohemian vibe while coastal/great lakes areas seem to vibe more yuppie. Personally, I think street art and independent coffee shops are a good barometer, and the twin cities are leading the pack in the midwest where that's concerned.