Hi Sahil, one is worst than the other. As a diabetic Sugar is your enemy so the regular one is really bad for you as it contains HFCS which will have very bad effects on health. Read on HFCS it makes you put on lots of weight and creates insulin resistance. Diet Coke and Coke Zero both use Aspartame and asculfame K which harm your brain and body in many ways including increase your risk for strokes. I don’t drink any soda. If I want I make my own flavoured Sodas at rhymes home.