In which parts of the USA do the say "soda" or "pop"?

Depending on where you go in the world, some people will refer to a carbonated beverage as "soda" while others choose to use the term "pop." For example, "Can I get you a soda" vs. "Can I get you a pop." I assume they both came from "soda pop" and were shortened at some point.

My question is if one is considered more correct or if they are both on equal footing? Also, are there certain parts of the US where it is just incorrect to use one or the other?

It's completely regional. You will find, in addition to "soda" and "pop," such terms as "soft drink," "coke" and "tonic." Note that "coke" in this instance is used generically to indicate any fizzy soft drink.

"Do you want a coke?" "Sure, thanks." "OK, what kind? We have Sprite, Coke, Diet..."

Here's a map that breaks down some of the regional variation: Pop vs Soda Map

The preferred use of Soda/ Coke /Pop is a regional difference, but it is not an issue of correctness. That being said, using the non-preferred variant in a region can cause communications failures. Being from the south, I initially used Coke during trips north and occasionally ended up receiving Coca-Cola Classic when I was really look for a listing of available carbonated beverages.

If you're looking for a generic solution that will be understood, I believe soft drinks is the most common term on menus and the like, at least in the US, although certain non-carbonated beverages that I wouldn't describe as a soda may also fall under that heading, e.g. lemonade.

According to phrasefinder

A soda in the US is called a pop in Canada.
In parts of the US it's called pop, not soda.
In some areas soda is also called tonic.

In the UK, we'd be most likely to refer to it as a fizzy drink.
Soda we reserve exclusively to refer to unflavoured carbonated water

Unless you are in Scotland, at which point it is juice, even if it has never seen a fruit in its chemical factory sourced life.

Or Ginger! On the West Coast of Scotland they quite often call any fizzy drink ginger (even though they rarely drink ginger ale/beer...)