Ascorbic Acid, Calcium Propionate - Differences?
As you're asking for "better", well:


better would be to use no preservatives and combat mould contamination in your bakery by using other means like an improved sanitary process.  


However, as you're also asking for pros and cons in a bakery environment as well, chemically speaking L-Ascorbic Acid is... well... an acid, so the advantage of Calcium Porpanoate is that it allows bread to be stored for the same time as with using L-Ascorbic Acid but without the faintly acidic taste that goes with an acid and that it combats mould better than L-Ascorbic Acid.