The vertical nipples have a metal rod the full length and that rod conducts heat. I haven't had one freeze up ever. Now, granted I use a de-icer and the bucket is indoors so not out in the wind. It also doesn't get colder than maybe -2F once every five years here. But, I'm not understanding how positioning the rod horizontally makes it less prone to freezing. It's still just a metal rod in a plastic housing, right?
Because the vertical usually has a drop of water hanging from it that freezes while the horizontal does not. The horizontal is usually "dry" when not in use.