Dogs tend to digest and absorb the good effects of C via different sources dependent upon existing health problems and their own digestion rates.
Ester-C is better than Ascorbic Acid (your drugstore "Vitamin C") because being neutral PH & non-acidic)it is less like to cause indigestion or diarrhea and because it is time released. But then there's the ongoing question, if your dawg completely digests-to-waste a meal within eight hours, how much of this very soluable form of C which is generally not stored is actually taken up ?
Calcium Ascorbate is better for dogs who are prone to indigestion and diahrrea as it is completely water soluable and flushes through their systems quickly, however that means adding it to each meal.
Sodium Ascorbate on the other hand sticks around and is easily absorbed.
So, in answer to your question ~ it depends on the dawg.