My daughter tried one of the variable NDs. It had lots of issues with "waves" appearing in the image from time to time. My quick research online revealed that this is a common problem with variable NDs. I purchased a 10 stop fixed ND from K&F Concepts. They are neither the most expensive, nor the cheapest. Several years ago I was given a set of bottom end filters from Amazon. They very negatively impacted image quality, and were useless in my opinion. I have been happy with the K&F, although there are other brands that I am sure perform as well for a similar price point. I got the magnetic mount, and love that mounting system. I can install or remove a filter in seconds, and don't have to remove the hood from the lens.
FYI, the variable NDs use two circular polarizing filters that cancel out varying amounts of light depending on their orientation in relation to each other.
Also, I picked the 10 stop because I wanted to get shutter speeds in the neighborhood of 15 seconds to a minute in daylight. I use this to get that fog like look to moving water. You may find a lower stop filter may suit your needs better, or perhaps not. One nice thing with a 10 stop is that you can adjust your camera for 1/30 sec without the filter, and then when you mount it, the corresponding exposure will be 30 seconds. No need for cheat sheets, or counting down stops on your fingers. At 10 stops reduction, I found that I had issues with meter reading in lower light with the filter installed.
Here's a shot taken with the 10 step ND. Shutter speed was 30 seconds, ISO 64, F16. I could have gotten the same shutter speed with a 6 stop ND, although not in full sun. The image I took of this scene just prior without the filter was shot at a meter reading of 1/60, F8, ISO 64.
