well i think that there not as close as one would think. here is how i like to think of it.
i mean with zoro its like, your being intimidated by his murderous intent, your fearing him more than your intimidated by him. your facing the aura of death around him, its like your looking in to the eyes of the grim reaper.

with luffy its like your being intimidated by the sheer force of his soul and will. its like a pressure barring down on your head, your not really in fear but in shock in a way.

at least that how i think of it in my pinion.