Coke has a mellower flavor, it mixes better than Pepsi. Pepsi is pretty outstanding on its own. (I always thought RC was a poor third, but my redneck mama loved it.)
You have to live south of the Mason Dixon line to stomach RC.....much akin to the taste for "sweet tea"
More:
http://exposingtheleft.blogspot.com/2005/05/pepsico-thinks-we-re-all-idiots.html