This has led to an interesting personal observation. While my preferred cola drink is generally Coke, with RC second and Pepsi third, I find I prefer Diet Pepsi to Diet Coke. On the other hand, I prefer Coke Zero to either. In fact, were I to rank my preferences for caffeinated diet colas, they would probably run: Coke Zero, Diet RC, Diet Pepsi, Diet Coke with Splenda, Diet Coke.
You might ask "why not get coffee?" That answer is fairly simple: I've never developed the taste for coffee. This is only odd if you realize that my parents are both frequent coffee drinkers (although not as much now as when they were younger). However, since none of my siblings developed the taste for coffee, I'm not completely out of whack. Oddly, all of our spouses drink coffee, at least occasionally -- robot_grrl once every week or two.
Of course, you might also ask "why not quit drinking caffeine?" That is also easy, I'm addicted and also use it to self-medicate my ADD.