I always prefer coca-cola to pepsi. If you ask me the reason, I have none. Maybe this is just out of habit. However, according to market research people like to drink coca-cola more than pepsi because of the design. People are easily attracted with red color compared to blue color. Coca cola brand color is red and pepsi uses blue.
It might be because Coca-Cola is a more popular beverage than Pepsi. Pepsi is popular too but it's never going to bigger than Coca-Cola. It's like when other online stores are trying to compete with Amazon's popularity. It's the same with Coca-Cola and Pepsi.