It depends not only on what you want to control, but also how you want to control it.

I’ll use a fader/slider for when I am recording MIDI and need to return the fader back to very specific spot quickly, or, if I want to “play” something like the cutoff freq on a filter. I can just smack the slider back and forth quickly like I am strumming a guitar.

I’ll usually map knobs when I need very fine controls. If you don’t need to min/max whatever setting you are working with, you put the knob about where it should be and only work a few degrees of rotation. (Rapidly min/max’ing a knob sucks since we typically do not rotate our wrist to mix/max quickly.)