Using circular, screw-on, ND-grad filters is not recommended as you cannot alter the positioning of the transition relative to the frame.
I prefer to use rectangular filters in a holder such as the Cokin or Lee systems.
Personally I use both Lee (outlandishly expensive) and some "Chinese Cheapies" that cost about 5% of the Lee prices.
Paradoxically, in terms of colour cast, the cheapies are better than the Lee - but that is pretty inconsequential as I created a preset in Lightroom to correct the Lee error and it is now just a single click to remove the cast.
My suggestion, therefore, would be:
(a) Opt for a holder system rather than circular filters.
(b) Choose a system to suit your budget and correct any colour cast in Lightroom (or ACR).
I assume that relative prices reflect relative quality in the manufacturing process but bear in mind that, in landscape photography (as opposed to, say, architectural work) a small amount of distortion is undetectable in clouds, waterfalls, rock formations, trees and grass. I have certainly never found that landscape images are degraded in any discernible way by the use of inexpensive ND or ND-grad filters.