It is not the carbination of the product which you need to be concerned with, rather the following ingredients:
phosphoric acid
potassium sorbate
sodium benzoate
sodium citrate
ascorbic acid
In the majority of products, diet brands will have a higher caffeine content than the regular brand.
The caffeine will create an acidic condition in the stomach which may lead to GI problems with higher consumption over longer periods of time. The phosphoric acid will lower the pH value of your saliva and in order for your body to compensate for this, it pulls calcium from bone (including teeth) to act as a buffer. The potassium sorbate and sodium benzoate are used as preserving agents against yeasts and fungi. The sodium citrate is used to keep the acidity level of the drink lower than it otherwise would be. The ascorbic acid is used as an anti-oxidant.
Sodas or diet-sodas have no nutritional value whatsoever and any calories which they may contain are classed as 'empty' ones. It just doesn't make sense to pay for and consume a product which has the potential to remove more from your body than it delivers.
Stick to water.
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KP -- Fitness Basics