First, you need to know for sure what the "ball bearings" are made out of. They could be totally unsuited.

Second, what is your criteria for "best"? Lowest cost? Greatest light reflectivity? Fastest? Produces least waste? Waste is easiest to dispose of? Steel pins (or steel ball bearings) will produce a high gloss finish, but can also so strip the case of residue that it takes more force to size cases and they can be more liable to stick. Dry media will not produce as glossy a finish, but it can be done quickly, sorting the media out can be quicker, and there less danger that you are producing a by-product (used media/used solution) that requires special handling or disposal procedures.