Pepsi - people always tell me they're the same and to stop being fussy but they taste different.
They taste different because they ARE different!
Coca Cola was invented before domestic refrigerators were available, so the drink has been made to taste good at room temperature. Pepsi was made later and so was able to make use of the cooler temperatures to make a better tasting drink.
So there's your difference. Coca Cola is best at room temperature, if you want something more cold and refreshing, choose Pepsi!