I know for a fact that Diet Coke and Coke Zero are exactly the same but under different labels. I'm guessing Pepsi did the same thing too.
For some reason they felt that Diet Coke was being marketed towards women, so they make Coke Zero to sell it off as more manly.
I drink any though. Usually prefer Pepsi because it has more taste, Coke hurts my mouth with too much fizz D: