Coca-cola was invented before the widespread use of refrigeration, and was therefore formulated to taste good while warm.
Pepsi, on the other hand, was invented later, and was formulated specifically to taste better cold.
Why does this matter?
If you think back to grade school chemistry, heat is just energy vibrating molecules. This vibration allows your tastebuds to “sense” more of the flavor. Therefore, the cooler a food or drink is, the more flavoring needs to be in it in order for you to taste it as compared to it being warm.
So, Pepsi, being made to be drunk cold, has a lot more sugar in it than coca-cola, because coke was made to be drunk warm.
This is why cold Pepsi can taste better than cold Coke, which will taste a little blander, but warm Coke will probably taste better than warm Pepsi, which will be way too sweet.
TL;DR: THERE IS IN FACT A LARGE DIFFERENCE IN TASTE BETWEEN COKE AND PEPSI, BUT WHICH ONE TASTES BEST DEPENDS ON THE TEMPERATURE YOU’RE GOING TO DRINK IT AT
