For NDs, I would take a fixed stop ND than a variable ND. For Fixed stop NDs, I would say either go for Hoya, B+W or Lee. I believe Cokin do not have 6, 9 or 10 stop ND filters.
That depends also on your usage. Cokin slot in allows quick changing and the combination with GND of the same brand (or the cheaper copy from Tianya). You will need the holder and the adapter ring for your widest lens, plus step up ring(s) according your lens(es).
Fixed screw in filters take more time to change, you will need the step up rings as well but for any GND you will need the holder and ring anyway.
According Filters for Photographic Lenses - Creative FIlters - www.cokin.co.uk Cokin has ND2 (1 stop), ND4 (2 stops) and ND8 (3 stops) types. With a 3 slot holder and 3x ND8 you can achieve 9 stops maximum.
For NDs, I would take a fixed stop ND than a variable ND. For Fixed stop NDs, I would say either go for Hoya, B+W or Lee. I believe Cokin do not have 6, 9 or 10 stop ND filters.
So far my vari-ND filter can only give me up to (at the most) 5 stop before the 'X' effect came in... and so in the end, I gave up and buy myself a Hoya ND400 and thats it.
Ans as mentioned by Rhino.. Also to note, inferior grade Vari-ND filters may even give you lesser than 5 stop before the X comes in.
Another point to note about the stacking of 3 x ND8 mentioned by Octarine, you will likely get some form of color cast. So, it's still better to get those numbers that you think will serve you better.
So far, I see 6 and 10 stop ND as more useful to ME. *I don't own a 6 stop ND, but many times I wish I had that instead of the 10 stop which I own.*
So far my vari-ND filter can only give me up to (at the most) 5 stop before the 'X' effect came in... and so in the end, I gave up and buy myself a Hoya ND400 and thats it.