Obviously I meant the stone sharpening, not using the knife steel. And as for the material, many sources say that plastic actually contains more bacteria and that end-grain wood is more gentle to a blade than plastic. On the other hand plastic is easier to clean when it's new, but it's also more prone to cuts than wood and they quickly get dirty (at least in my limited experience). IMHO every board needs to be scraped or sanded from time to time to remove the top layer completely, that's the only way to keep it clean.