Coke all the way. Pepsi is okay sometimes, but it's much sweeter. As for Diet Coke or Diet Pepsi, they're both disgusting. I've never liked the diet taste. And Coke Zero, equally gross. No it does NOT taste exactly like regular Coke.
As for preferences being 50/50. I know about 2 people who prefer Pepsi, everyone else prefers Coke.