Hmm...
All throughout the north-east, you get pop. I've lived in Monterey and San Francisco, and the only ones there who said "soda" were the little kids. Probably from watching TV.
In Alaska, everyone I've met either says pop, or an actual name brand.
But then again, not many people refer to the north west when they say "the north".
"Soda" is a very general word for the beverage, used in nearly all locations, although from my times spent in the south (all south) if I do hear "pop" it's attached to "soda", "soda-pop". I've never heard anyone in the south say "pop", unless they're a visiting tourist.
Maybe yer talking about the mid north?
Okay, I apologize. To the 12 people in North Dakota, forgiveness please.