I think it is a Midwestern term. When I lived in Michigan it was always "pop."
When I first moved to California, 30 years ago, we went to a restaurant and I asked what kind of pop do you have. The waitress said, "you're not from around here are you?" It's been soda ever since. Now, when I hear my family in Michigan refer to "pop" it sounds so weird.
TIS
"Mankind must put an end to war before war puts an end to mankind. War will exist until that distant day when the conscientious objector enjoys the same reputation and prestige that the warrior does today." JFK
It's been soda ever since. Now, when I hear my family in Michigan refer to "pop" it sounds so weird. 
