Isn't this completely fine? If you and I both make aspirin, but we both put a little something extra in it (me vanilla, you salt) and I put banners on my web page saying "bastawhiz's salty aspirin puts the ass in aspirin", doesn't this just seem like typical rivalry? My point here is that defamation is defamation no matter the scale. I think scale is relevant re: damages, but not as to whether or not rivalry escalated to defamation in the first place.