I don't want this to get into a debate between the two colas, really I don't like either of anymore, not for a long while, this is about a phenomenon I encountered in the 80's, during my University days. I even wrote both companies about what I had found out, and only one wrote back and thanked me for my interest in their company.
What I found out, and this was through an informal study, and granted that it was less than 10 people, was that, for those who displayed a preference (and I think the cola wars have long since subsided, certainly it's not as evident as it was in the 80's), if they drank Coke, they also drank 7-Up; conversely, if they drank Pepsi, they drank Sprite. Yet, Coke owns the Sprite brand and Pepsi owns the 7-Up brand. So, you might have to go to two different vending machines or stores in order to get both. The idea that I had, was for both companies to maximize their profit, assuming my small pool was a reflection of the greater populace (and I grant that this pool was in a relatively small University city in Canada), was for them to trade brand names (Sprite for 7-Up in a straight deal). I think mostly they laughed at the notion, but I still wonder about whether this was a local phenomenon or what.
For the record, I preferred Pepsi and Sprite.