They are very similar. However, one key difference between the two species is their leaf shape. Silver birch leaves are thin and triangular, while downy birch leaves are broader and more oval-shaped. Silver birch leaves also have a more shiny, metallic appearance, while downy birch leaves are a matte green color.
Furthermore, the texture of the bark is slightly different also. As its name suggests, downy birch has a softer, downy texture to its bark, while silver birch has a more papery and fragile bark. Silver birch bark is also more silver in color, while downy birch bark is more brown or gray.