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http://blogs.mathworks.com/steve/2006/10/23/nonflat-grayscale-dilation-and-erosion/ has some details about the non-flat structuring elements. In one line, flat strels have a common weight for each pixel in it, and non-flat strels are weighted. The 'ball' strel in particular has highest weight at the center, and tapers down uniformly towards the edges. As to which one is better, I suppose it is application specific.