Hi skopio,
yes, basically the "P" system is larger than the "A" system.
You can check out their website. Just search YAHOO for the words "cokin" and "filter". They have pictures that describe their filter system, and this comes with charts that show the difference between the 2 systems.
For a hands-on, go down to Cathay Photo. They carry a large range of Cokin filters with the relevant accessories. Try it out and see if you like it before buying....one man's drink is another's poison.
I have been using the Cokin system since 1991 ;p Personally, I find it very convenient. I will probably pop down to Cathay Photo this weekend to purchase a 62mm adapter ring ($15) for my new lens. With this system, one only needs to buy an adapter ring to match the diameters of the lenses one has. The holder and filters are not affected by the change of lens (unless the new lens diameter is bigger than the filter size, that's why "P" system is better because it's bigger).
I'm pretty happy with their Circular Polarizer :thumbsup:
What sort of Neutral Density filter are you thinking of, and what kind of shots are you interested in shooting with the ND filter?
Bernard