A pure full ND just allows you to slow down your shutter speed. As dynamic range improvements has come in lower noise rather than higher full well capacity there's no difference here, to slow down your water you still need NDs.
Then you have graduated NDs which are used to darken the sky so you can expose the foreground brighter. Here increased dynamic range has been helpful, as you can underexpose the foreground more than before. You'd have to use Sony's sensors (which Nikon too uses) to get any significant difference, a Canon still don't have that good DR. Of course, you can also use HDR techniques, ie shoot one bright and one dark exposure and merge in post.
I have myself a Lee Seven system to my medium format tech cam (a Linhof Techno), I use it for grads only. I have a 3 stop ND and a polarizer too but as a normal 72mm filter (you can attach the Lee Seven in front). I use a special technique where I apply the grad when I shoot (to capture good overall exposure of the scene), and then make a LCC shot with the grad on and cancel out it in post, and apply new tonemapping in post (which actually often becomes just a grad, grads are still good to get a natural look). In really difficult situations I resort to bracketing and do HDR merge, but surprisingly often a grad is good enough and I do prefer the "one shot capture" when possible. Even if the horizon is uneven or a tree shoots up over the horizon those elements often doesn't suffer too much from being darkened, especially if you choose to have a grad effect in the finished picture.
Still it should be said than grads are not so popular these days, most are satisfied with the possibilities provided with increased DR in the sensor and/or HDR merging techniques. It's somewhat a matter of taste, I find it quite enjoyable to use grads when shooting.
With pure NDs you can find high quality glass filters which are multi-coated to minimize reflections. Get those.
With graduated NDs there's bad news. As far as I know there are no multi-coated filters. There are glass filters primarily made for the movie industry (Schneider has a series sometimes used for landscape) but they are very large heavy and expensive, and still not coated. Lee filters are resin filters but of high quality, uncoated as everyone else. Low quality resin filters can actually blur the image somewhat (been there, done that), so I recommend to get Lee or any other of equal quality if you get graduated NDs.
The lack of multi-coating makes the graduated filters a bit more reflection-prone than they otherwise would be. Reflections occur easier if the incoming light comes at a low angle, ie if you shoot wide angle, and it occurs more easily if you have stacked filters (such as UV + grad) or if you have dust or grease on the filters, or the filters are not properly shaded. In really sharp light, say midday light with bright high contrast features in a building or so you almost certainly will get reflection issues, so in those light conditions graduated filters are not really usable, then use HDR-merging or wait for a better light condition. In a regular sunset and sunrise with clean filters it will work fine.
