According to Steve Spangler, he used Diet Coke not because it’s more reactive, but because it didn’t contain sugar , and thus made cleanup much easier than with regular coke.

Found it, and I was slightly mistaken:

Why should you use diet soda? The simple answer is that it just works better than regular soda. Some people speculate that it has something to do with artificial sweeteners. More importantly, diet soda does not leave a sticky mess to have to clean up. Hey, that’s important for your clean up! [Source]