Even though it's been heavily processed and pasteurized, prune juice still has a high amount of antioxidants. The reason for this is plum fruit's intense purple pigments, which are a good source of phytonutrients referred to as neochlorogenic and chlorogenic acid. In fact, if you choosing between prune juice vs. apple juice, you're going to get almost 75% less antioxidant content with apple juice!
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010