/3/ - 3DCG

>>593788
One is easier to control with pressure, easier to keep consistent and lets you keep your brush "sharp", the other works by building up strokes and almost immediately makes everything blurred and fuzzy. I'll let you figure out which is which.
Flow just layers the brush stencil over the top of itself. Opacity (in the Photoshop/decent brush engine sense) sets the opacity of the stroke to the set value or pen pressure so long as the opacity of the underlying stroke/brush stencil is lower, regardless of what the previous opacity is. Worth noting that once you let go of the stroke, in most software, it stops doing this, and layering additional strokes adds opacity.
Flow is useful in a lot of situations but I wouldn't ever want it to be the one I'm stuck with. If you've got a background in art and ever tried some of the less popular software like early OpenCanvas, you'll know what being stuck with flow feels like: a lot of half-baked art software uses flow but calls it opacity. It's instantly recognizable, and if you don't know how Flow functions you'll probably just think of it as "shitty Opacity" since it's just the software crapping out brush stencils at a set rate and you'll immediately notice that the default brush is "just a bunch of circles" instead of a continuous line when you use the software's opacity.