It seems like it's really a regional thing. Down south, everything is a Coke. In the midwest it's pop. In New England, I think it's soda. I'm not sure about out west though. I've never been there before to notice.
In Arizona it's soda...if someone were to say "Pop" it would sound extremely old fashioned. Cola would sound out of place, even when referring to a Coca Cola which is just referred to as a "Coke".