Not necessarily. Diet Coke just has fewer calories and fat that prevent you from gaining more weight. Someone who drinks Coke on a regular basis and then switches to Diet Coke may experience a very small weight loss.
Nope. I did a research paper for psychology class about diet soda making people fat. I've seen at restaurants people ordering Big Macs and super huge fries and then get a diet soda. It doesn't matter what kind of soda you drink, it depends ont he calories you intake.