Longshadow wrote:
The two filters have two different functions.
If you want to be able to decrease shutter speed (ND) and eliminate reflections (CP) use both.
Just be aware that having both on a wide angle lens could cause vignetting.
Note that the CP will reduce the amount of light entering the lens, the ND will provide an additional reduction.
If you just using the CP to boost colors and not remove reflections, leave it off. You can always boost the colors in post and possibly avoid vignetting.