For consistency reasons it's better to stick to one selected guideline instead of mixing. You need to be consistent across the interface, but also consider the device it was build for.
So, you mentioned Material Design, so I will assume that you are following that guideline and maybe are creating Android/Web apps. In that case, you should use the vertical icon. You can see example images with the icon in the Components > Contextual Menus
Now if you were to design for Apple devices, their guidelines use the horizontal icon, see System Icons, especially the More icon.
I prefer the horizontal icon, since it's reminiscent of the typographic ellipsis which was intended to suggest "more" items, so it has a more common interpretation in the user's minds, but above preferences, I would go for consistency.