I'd just like to know why STL and the central IL region is so deep in saying "soda". Granted, any time I've headed up to Chicago I don't think I've heard "pop" so I'm not sure how ubiquitous it is in most of IL even including Chicago. But having not grown up there I can't say myself.
Anyways, it's definitely soda in most of IL. Don't think I've ever heard it called pop there, other than by my grandma who was from the southside of Chicago.