I'm not sure about Diet Soda, but Lo Carb bars stall me out. I don't drink diet soda that often because of the Aspartame that the Atkins program says can stall you. So, I have them as a treat. Went out to a restaurant the other night and had my first in about 2 weeks, and boy was it good! I should have saved it for dessert time!
I think everyone is very different in how these things affect them. You should experiment and see how it affects you. Cut them out for a week, and see how you do and then add them back slowly and see how your body reacts.
G