The primary difference between a small silicone butt plug and a small metal butt plug lies in the material used. Silicone is a flexible, soft, and body-safe material that can be more comfortable for beginners due to its pliability. Metal plugs, on the other hand, are rigid, heavy, and can provide a more intense sensation due to their firmness and weight. Metal plugs also have the added benefit of being compatible with temperature play, as they can be warmed or cooled before use.