Water is generally better than either form of soda.
BUT ... PHYLODIA has made a great healthy step in moving from sugared soda to non-sugared. The rumours and myths about artificial sweeteners are not backed in science. What we do know at the moment, is that if you're going to drink any soda at all, diet is better for you than non-diet. I don't think we should be slamming that choice.
Let's keep it friendly and supportive! :)