They are for completely different purposes and have completely different effects.
The ND filter is to slow everything down. Say you want to get some motion blur, but the sun is shining bright and your slowest possible focal length is say 1/60. That will not blur much. Add a four stop ND filter and you now get a shutter of 1/4. A graduated ND filter is a very useful tool. It has one side with the ND and the other side clear. It is for when your scene really needs two different exposures.
A CP filter removes polarized light. It is most effective when the sun is low in the sky and you are shooting either 90 or 270 degrees from the sun. It will cut reflections off windows, reduce glare off moving water, etc.
The Cokin system is a good system for the price. It allows for moving the filter up/down and rotating.