There is a distinct taste difference between the two. Some prefer coke and perhaps even dislike pepsi.
Personally i think pepsi max/diet is disgusting compared to diet coke or coke zero . Though i don't think normal pepsi is bad but i too prefer coke way more . So i get disappointed when i am told "is pepsi ok?"
Its the common response from restaurants and take away when you ask them for a coke. They probably serve pepsi because its cheaper but they still charge same price.

Fun Fact#1:The owners of Coca-Cola are the same ones for Pepsi.

Fun Fact#2:Pepsi literally is watered-down CC,they are made from the same "syrup",wich the base ingredient of the two.