Neutral density filters just make things darker without affecting color, so you can use slower shutter speeds...Polarizers actually block out certain types of light waves, namely light that is reflected, because it has different properties than diffused light, so colors are more saturated and reflection and glare are minimized. So using a polarizer, you also get a darker image and can use slower shutter speeds. You can even use them both at the same time if you want it really dark...