I find that they taste differently.
Here’s the list of ingredients in Coke Zero Sugar:
Carbonated water, caramel color, phosphoric acid, aspartame, potassium benzoate, natural flavors, potassium citrate, acesulfame potassium, caffeine,
Diet Coke is missing two ingredients listed above: potassium citrate and acesulfame potassium. Acesulfame potassium is a calorie-free sugar substitute and potassium citrate is a common additive in beverages.