Soda or pop? It ultimately depends on where you live. Forget accents and other colloquialisms, some say the best (and most fun) way to determine where a person comes from is how they order a soft drink.
Ask for a pop in on the East Coast and locals will know you're visiting. Likewise, if you think you're getting a soda in Nebraska.
And when ordering a drink in many parts of the South, don't be surprised to hear the server ask "What kind of Coke would you like?" - regardless of brand.