Difference in quality among Cokin P Series and Z-pro filters and also other filters

[max11]

Hello everyone.
I've been reading a lot of forum posts and websites about filters.-

I learned that for filters (as well as lenses) you get what you pay.

I arrived to the conclusion to buy a CPL screw-in filter (possibly from B+W,Heliopan, Hoya Pro1) for the 17-70 (as it should be nearly useless for the 12-24), while I'm going to buy a Cokin holder for square system for Grad ND and ND to be used mainly on the 12-24.

As far I read, Cokin filters are not very good (both because made of resin and for a cast effect), while Lee, Schneiders, Tiffen,Singh Ray, Hightech are much better.

Cokin holders are available as P and Z-pro series. The first one with an adaptor shouldn't produce vignetting at 14mm (I am not sure what happens at 12mm and even at 10mm).

The difference in price between P and Z-pro is relevant: considering the kit holder + 3 ND filters, the cheapest I found is £ 40 for P Series and £115 for Z-Pro
Is this difference in price due only to the dimensions or also to the quality of filters? Are the Z series cokins filters as bad as the P ones?

Also because the single Cokin filters cost much more for the Z (£36/50) compared to the P series (£10/16).
I couldn't find it out neither on Cokins website.

I could decide to buy from Cokins only the holder, and then another brands for the filters. Any suggestions?

The Lees filters are 100mm and fit the Cokin Z-Pro holder.
The HiTech are 80mm and fit the Cokin P holder.

Which other lenses fit the Z and which one the P?

thank you a lot