I'm from the heart of "pop" country and I say "pop." But I don't like it much and never buy it, so I don't say "pop" often.
I didn't know anyone said "soda" until I went to the East Coast for the first time.
The tendency of Southerners to say "coke" as a generic term for "pop" has puzzled me since I first heard of it, because it would seem to create unnecessary confusion. Down there if people ask you, "Would you like a coke?" does this mean you won't know if they are asking if you want generic pop or specifically Coke?
Saying "pop" could lead to confusion. Does my pop's pop refer to his drink or my grandpa? However, I've never actually seen such confusion arise.