I have SPDs on my mountain bike and Look Keos on my road bike.
The shoes and pedals on the mountain bike are almost exactly double the weight of those on the road bike; SPD shoes have very thick dense rubber on the soles to create the well for the cleat. Road shoes have no treads at all, just a rigid plastic sole. Road shoes and pedals make a fantastically comfortable and flat platform when you are out of the saddle.

