Original* Coca Cola (i.e. "Coke Classic") has notes of vanilla and cherry mixed with the cola flavor. A Cherry Coke adds additional cherry flavor to accentuate that note, but the vanilla remains. The reverse is the case with a Vanilla Coke. Dr. Pepper is more like a Cherry Coke, but the vanilla note has been removed. The caffeine adds another flavor note, plus it is a stimulant (which tends to hook frequent users, similar to the effect brewed coffee has).
The Coca Cola Co. website is upfront in its FAQs, saying that a 12 oz can of Coke Classic contains 34 mg of caffeine, and a Diet Coke has 46 mg. Dr. Pepper's website isn't as forthright, I can't find any notation as to their caffeine content, which makes me suspect it's significantly higher.
* What's today called Original Coke really isn't. Over the years they've tweaked the formula, most noticably when they did the "Old Coke/New Coke/Coke Classic shuffle" back in the '80s. That was also when they substituted high fructose corn syrup for cane sugar. The taste has never been quite the same, though most consumer either never noticed or have long ago forgotten. If you want to truly experience what original Coke tasted like, try and get your hands on Mexican Coca Cola in the 12 oz glass bottles, which is closer to authentic and still contains cane sugar. (I know this is a little off-topic for a radio discussion, but give me a pass this one time.)
