Diet sodas are better only in the fact that they do not have sugar in them. Even diet soda can be bad for your teeth due to the acidity caused by the carbonation. It is OK to drink sodas in moderation. The damage that they can cause is directly related to the time your teeth are exposed to the liquid. Therefore, do not drink sodas all day long. Try to rinse or brush afterward, and limit the number of sodas you consume each day.