The green bullet and similar mics are for the Little Walter/Paul Butterfield style, where you cup the mic in your hands and go through a guitar amp in order to get the resulting distorted sound. There is really no way to get that sound otherwise. Harpists who play this style generally like cheap mics - some even go for crystal or ceramic mics. You would obviously destroy a condensor or ribbon mic with this technique.
But if you want the "natural" harmonica sound, go for a mic which has even response and a smooth high end, at a distance of at least a foot away.