It doesn't matter. No place (talking restaraunts and the such) sell Coke AND Pepsi, so if you order one of them you get what they have.
In almost every case, the server asks "Is <oposing company's cola> okay?" I don't mind either one, but I'm able to tell the difference between them. If given a glass of Coke, I'm pretty sure I could tell you it was Coke.
The only reason a resturaunt is constricted to one company's soda is because they sign a contract saying they will only sell their product. If they don't have a contract, they can sell whatever they want.