What You Did
You made a cola fountain. You timed the mixing of different chemicals to get an explosive result.
When the Mentos candy hit the cola, it bubbled so much and so fast that it exploded out the top. The diet cola reacted more.
A bottle of cola that wasn’t opened recently won’t bubble as much, if at all.
Use a different kind of soda pop. Try different brands of the same kind of soda pop. See if the temperature has an effect on the reaction of the cola. Compare the reactions of caffeinated and decaffeinated colas