The only thing I would look for is a magenta color cast on the darker B+W's. Schneider makes the glass for most of B+W and I have
seen some magenta casting issues. This is easy to test by putting the filter up against a white sheet of style, that allows light some
light pass through. Their built quality is the best. If you are going to use the filter a lot, then you might check to see if Heliopan makes
one as they use brass in their construction which is stronger and won't bend.
With any good step down ring, you should be fine, no light issues, brands like B+W, Kalt, Bower, and Heliopan. I prefer Heliopan since
they use brass. For some reason it just seems that the brass rings are easier to get off, the aluminum ones seem to want to screw on
too tight and once on can be very hard to get off. Occasionally I will tap a stuck ring which many times will get it started to free up.
ND 3.0 or 10x is a lot of darkness, I have the High tech 10 stopper which is resin. No color cast issues with it.