Short answer: They are similar, but there are some key differences, and whether they are important for you or not, will depend on your photography goals.
- NN6 is much newer design than the M series, more lightweight, less heavy, easier to carry around, the upper rotator has 7.5 degree positive click stops that can be disengaged for free rotation, there is a camera plate included that supports many cameras, stop plates are included for remembering the no-parallax point, comes with RD10 rotator (10 detent click setting options).
- M Series is more robust, more stable, supports more weight, supports longer telephoto lenses and heavier cameras, requires a separate Arca plate for the camera which is not included with the product because many photographers getting this product already have it, the upper rotator has 7.5 degree click stops that cannot be disengaged for M1 version and it has a free adjustable friction rotation for the M2 version. Stop plates, nadir adapter and camera plate can be purchased extra, you would also need the 6mm spacer adapter
to support a fisheye lens to avoid rails being visible in the picture.