hey, I'm happy to help you with this since I draw b&w with screentones since forever.
I normally use about 3-5 different tones (must have one for general shadows of characters/things, other tone variants are for "colors" of things"... since it's b&w you must have a few tones to make it less flat i guess.
I have both and they are quite the package with tones and materials you can use and customize, you can download some for example, blood stains or smokes or explosion tones in case you need them for our specific scene
the more you have in your library the better
just my personal thought, half tone will save you a lot of troubles and ink since it's all black and you can avoid uneven color distribution like when you color it with solid color.
plain dots are usually for flat surface with no texture (at least it implies so), and grainy one is, for example, for textured surface like clothing etc
you can totally reference some manga to learn how to use them effectively
my work example