>> Yet, over time, get exposed to enough of those ads, and when it's time to buy, your hand instinctively goes for the one your brain is bombarded with ads for all day.
I don't really think that has anything to do with it. At work we having both Coke and Pepsi vending machines. Most people prefer the taste of one or the other, to the point where if a place only serves Coke and they prefer Pepsi, they won't order it. Not unlike beer in America (Bud Light vs Miller Lite, etc)
Soft drink choices come down to availability most of the time. Chances are if you are at an event such as a sports game or concert, they've already made the choice for you. Same goes for a restaurant, McDonald's has Coke and Taco Bell has Pepsi.