If I had to choose between the 2, I'd pick Pepsi. However, the best soda is Jones Cola Cream Soda. They're hard to find, but if you ever see any kind of Jones Cola soda, buy it and drink it; its amazing!
I don't really have fizzy drinks that much to be honest, I prefer juice, water or alcohol . If I were to pick though I would define go for Pepsi, it just tastes a lot sweeter and Coke is just too harsh in my opinion.