A round ball has the smallest surface area of any shaped object. However, there is a greater surface area/more drag on a round ball than a fish shaped or disc shaped weight with a fin once it is starts moving in a single direction. Should there be any forces to the side of the weight, ie. tidal flow across the path of the weight it could end up with more drag. My preference is to increase my weight, ie. from 12 lb to 15 lb, and maybe reduce my downrigger line drag than use a disc shaped or fish shaped weight.