unfortunately this debate gets clouded by bias imo.
if you are talking, simply, who looks better or who has the look more people would want then it's not hard to see how arnold is advantageous.
arnold had far greater appeal than mentzer based on popularity, status, inspiration, history and, of course, he had the show piece arms, chest and a bigger overall structure.
fortunately judgement of body building is a little more structured than 'appeal'.
if we are talking strict body building competition then it is not hard to see why mentzer has a very strong case.
aside from posing, bis and chest, mentzer blows arnold away on lower body, back, abs, tris, delts, traps, muscularity, proportion, symmetry and hardness. it's not even close.
this is why mentzer, other competitors (that day) and many other passionate hard core bbing competitors and fans were pissed, because the underlying statement made by arnold's win was, 'it doesn't matter how hard you train, diet, sacrifice or what genetics you have, the judges can be corrupted.'