Lee filters aren't available in the 84mm Cokin P-series size so you'll need the larger 100mm holder if you want to use the big stopper.
(ie. Lee Foundation Kit or Cokin Z-Pro).
Lee are expensive but flawless.
Colour casts are going to be the biggest drawback of cheaper filters. Cokin themselves tend to get a distinct magenta cast when stacked - can only imagine what the eBay jobs are going to be like. Personally, avoid at all costs.
Use of NDs and GNDs is going to be greatly dependant upon the lighting conditions and/or the desired effect you're going for.
GNDs are used to balance varied lighting within a scene where as NDs are typically used to allow for a longer shutter speed for blurring motion and the use of a wide aperture in brightly lit scenes.
View attachment 283778
2-stop ND.
I've got a B+W "Big Stopper" though it's a screw-in filter. I tend to avoid using it with my wider lenses on account of vignetting when stacked - functions perfectly with anything longer than 28mm (based upon Nikon FX).
View attachment 283777
30 second exposure using the B+W110 (10-stop ND).
Good luck!
