Actually, this has been studied and is true. There was a brief segment on it in Psych 101. Someone correct me if I've got the details wrong but it goes something like this:I don't think you're correct on the diet soda.
The sweetness in the diet soda tricks your body into thinking nutrients (sugar) are coming and it releases digestive enzymes into your bloodstream. When the enzymers aren't used they are converted to fat. For the same reason, diet soda actually has an appetizer effect and makes you hungrier than you really are.
That said, I only drink diet soda.