Neither. We reached out to several noted nutrition experts who shared our ambivalence about this issue. A 12-ounce can of regular soda has 140 calories, all of which come from roughly 8 teaspoons of sugar, usually in the form of HFCS. For a person who can manage that amount in his or her daily calorie allotment, that's not a significant number. For a person who can't, diet soda helps eliminate those calories, although there's not much evidence that diet soda is useful for weight management. Also, the long-term health effects of consuming artificial sweeteners are still unknown.
BOTTOM LINE Drink regular if you can account for the sugary calories in your daily intake. Drink diet if you can't. Either way, treat soda as a treat, not as your primary source of hydration.