Kombucha can be an aid to digestion. In addition to the beneficial bacteria, it also contains some acids and enzymes to aid in the breaking down of food.
Kefir Soda is more of a general probiotic beverage. While it does contain enzymes and acids, they don’t seem to have quite as strong an effect as those in kombucha. However, Kefir Soda contains a greater number of bacteria strains than those found in kombucha.
Both beverages are beneficial in aiding natural functions in the body, and both are great for hydration. Depending on your needs, consuming one or both is a matter of individual taste.