If it's purely a Coke vs. Pepsi, I choose Coke. More specifically, I took the Pepsi challenge 3 times during the '80's and chose Coke all three times. The testers were not pleased.
However if it's the Coke lineup vs. the Pepsi lineup, Mt. Dew. And not that god awful Code Red, Brown Note or whatever other crap they've come up with by now.