One of the most complex issues. Many people cannot distinguish between Pepsi and coke, but comparing the information from the Internet and my own feelings, I would say that Pepsi is a little sweeter, and Coca-Cola more bubbling. Also the Internet contains information that Coca-Cola are present fruity notes, and Pepsi has a rich taste of Cola. But let's not forget that the sense of taste may depend from individual to individual, and from the geographical region in which the produced beverage. The compositions of drinks depending on the region may change.
I will not go into this issue deeply, let me just say that at the present time the composition of the two drinks are almost identical. Coca-Cola does not fully disclose the formula of the drink, the composition of the Pepsi is also known as in 1923, the company has passed through bankruptcy and was forced to disclose this information.