Isoascorbic acid is slightly more active as a developing agent. This isomer is more commonly called erythorbic acid. The sodium salt of either agent can be used in developers and do not require added alkali as the acid does. Erythorbic acid also tends to be a bit cheaper. What does the developing is the ascorbate ion which is produced by either ascorbic acid (think hydrogen ascorbate) or sodium ascorbate when dissolved in water.