Typically drink water - just curious. What is more harmful over a few years - soda with corn syrup, or diet soda?

Both harmful. We don't yet know the full effects of high fructose corn syrup verses artificial sweeteners in diet soda. More time and studies need to be done. The obvious answer is to drink water and stay away from those soda drinks until more information and safety info becomes available.
It's the Acid. Sodas are the most significant dietary source of tooth decay. It is the acids and acidic sugar byproducts in sodas that cause decay and erosion of tooth structure. Since sugared soda provides dental plaque with substrates that turn into acid., the soda with sugar would be more harmful. However, diet soda also contributes to a high level of enamel erosion without sugar.