Finer grind = potentially technically more surface area, but you do want the higher ridges of coarse ground pepper. Otherwise your bark might end up "all in one piece", similar to using or spritzing with too much sugar.
Pepper always seems to make more of a difference to me than salt grind, but that could be the rationalist in me knowing that it should all just dissolve anyways. I usually just use MSG in my rubs as it is, versus coarse kosher salt