Thank you guys for sharing your experience!really really important for me.
I will try to apply it as soon the weather allow me!

Today I was reading "Understanding Expossure" by Brian Peterson and he looks giving more importance to the DOF and then to the sharpeness, at least this is what I have understood (or he says) until this part of the book (pag.56, I'm sure many of you own it).
After discussed of when we want and how to obtain increased and reduced DOF, then he mention the mid apertures. In fact he says: "I use often f/8 and f/11, but only when the DOF is not a concern".

All in all, I think it depends on what we want to obtain. A balance of a longer or smaller area of sharpness with the max sharpness possible, or sometimes max DOF at the cost of a perfect sharpness

I'm posting it as a summary (please correct me if there is anything not correct!):

When it comes to 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.

The choice is important, I guess, because if I decide to buy from Cokins only the holder, the filter brands I can chose among depend also on the holder.

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

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

Again thank you you are very gentle.
I would say much better and agree with you the difference is relevant!
The polarizer has been used on the R on the 1st photo and on the L on the second one, hope I'm right!

I would appreciate if you can ask the shop,maybe I will write them tomorrow.
on the same website the difference is between £39 and £115, quite a lot, I couldn't find out neither on Cokin website if there is a difference in teh quality of lenses.
I have asked specifically this on another forum, don't know if I can open a brand new post here to give more relevance to it.

Ah right! it looks a little bit complicated for me!

I modified the contrast,brightness and gamma (50 -8 1,5 and sRGB) to be able to see all tones of that page, but after a while your eyes start to suffer!
Is that normal?

This tool looks very interesting.

So which is the final recommendation?

calibrate or not?

I have got a laptop, is worth spending money on a Spyder or Huey software or better to try different labs until find the one that suits our colour?

Last week I started reading this website
and tried to calibrate the gamma, but got stuck in the middle of point 5

I think that is very important, very basic but very important.

So with the exception of the f/2.8 for the 50mm?

If I can add a point, I don't know exactly what (or at least all elements) makes a picture a good picture (rather than personal preference), I always liked the pictures that my point-and-shot randomly made with increased DOF (that is why i used to set f/38 when possible!)
but,if I understood the concept, we have to aim to the max sharpness (if we don't want specifically a different result) and then to the max compatible DOF or bokeh.

very good idea!

I prefer the 2nd for the reflections, the 1st for the sky, can't decide on the 3rd one, maybe the 1st....

Don't I like polarizer?! or is related to calibration?

yes absolutely a screw-in filter, also for protection!
especially if it has no point on 12-24, and will be used only on the 17-70 and rarely on the 55-300.

Didn't you say that you keep the filter often (in sunny days I suppose or if you want to cut the reflection) and then add the Cokin system if you need?

About the shop you bougt the Zpro kit, is online or a street shop?
thank you

Really thank you all and Stefan particularly.
I had read again the post and you repeated me some things 2 or 3 times without losing your temperance!
sorry but the subject is new and I'm learning it now!

To summarize:
A) I'll get the Cokin P system + grad ND kit (£40) I would be willing to spend something more for the Z-pro to get better quality and avoid the vignetting, but the cheapest I found (£135@Warehouse) looks to me too much at the moment, maybe because I have not experience on those yet.
B) I'll add a Cokin ND4 or ND8. (I can't choose!)
C) I'll buy a screw-in circular polarizer. I didn't understand why technically, but it looks useless for the 12-24


So I have two options for the CPL:
1)buy a 77 mm screw in CPL plus step up for 17-70 and 55-300
2)buy a circular polarizer Cokin (£56)
Which option is practically better?

Bravo! I was ignoring all this chapter and now start to understand the meaning of these graphs! even if it is quite complicated to rememeber which is the best F aperture for every focal lenght for every lenght, isn't it?

Also this post is interesting