OK, the following is strictly chemistry, with nothing related to experience in wine making.
Both ascorbic acid and k-meta are anti-oxidants, so both will prevent browning of fruits and vegetables. You may recall that in restuarant salad bars they used to spray the produce with a sulfite solution (Na-meta) to prevent browning, until the asthma/sulfite thing reared its ugly head.
There is a difference, however. Ascorbic acid in water (must) exists only as an ascorbate ion - C6H7O6- (I think that formula is right). K-meta in water, however, can exist in many forms. At the acidity of must/wine it is split between SO3-2 (sulphitic acid) and SO2 gas. I'd be willing to bet that it is the SO2 part that inhibits fermentation.
If I'm correct, then ascorbic acid is the perfect choice for fruit and sparkling wines, because it inhibits oxidation of the wine, while having minimal impact on the yeast.