What rots teeth more diet vs regular?

Answer 1

Diet rots less.

Here's how it works. There are two elements at work here: sugars and acids. The artificial sugar in Diet Soda contribute toward tooth decay, "real" sugar does. Artificial sweeteners are just the wrong "stuff" and not what cavities need to develop.

However, all sodas have acids in them, which wear on the enamel of the tooth. This doesn't directly cause tooth decay, i.e. it's not tooth decay just by itself, but it lets sugars, starches, and all else that would decay your teeth to do so more easily.

Neither kind of pop would do very much tooth decay if you drink them with a meal (so the soda pop's sugar gets taken off your teeth by saliva and food), drink through a straw (often achieving much less contact with your teeth!), or rinse your mouth out with water and brush your teeth soon afterwards.