i did do a search and PLs also reduce reflections and increase contrast...and for ND filter reduce the amt of light ...so the shutter speed can be reduce too ..and PLs also the same..
Main diff btn the two filters
CPL - will make skies appear deeper blue, will reduce glare and reflections off of water (can see the rocks or fishes of course if the water is crystal clear not an x-ray vision) and other surfaces like leaves, and will reduce the contrast between land and sky and bring out the cloud details too.
Nice videos on effect of using CPL
Here &
here
ND - This is useful when a sufficiently long exposure time( 1 sec or more) is not otherwise attainable within a given range of possible apertures (at the lowest ISO setting).
Situations where ND filters are particularly useful for the 'special effects' include:
■Smoothing water movement in waterfalls, rivers, oceans, etc.
■Achieving a shallower depth of field in very bright light
■Reducing diffraction (which reduces sharpness) by enabling a larger aperture
■Making moving objects less apparent or not visible (such as people or cars)
■Introducing blur to convey motion with moving subjects
A nice video on
ND filters
Read more
here