I grew up in southern Illinois drinking 'soda'.
I went to college central Illinois 150 miles away, and there it was always 'pop', both with the northern Illinois kids and the local students.
Our best friends lived in Nebraska and Iowa, and they were not consistant, either.
So, I think when you talk midwest US, you still can't generalize.
I order diet cola whereever I go, and even in Atlanta GA, land of coke = anything, they understood me.