As much as I admire the incredible physique of Arnold Schwarzenegger, I must admit that he did have his flaws. Maybe more than Sergio.
Arnold's upper legs were sometimes a little weak when compared to his upperbody. His left arm, in some shots, looked smaller when compared to his right arm - speaking from a straight-on shot, not a twisting shot which purposely made the left arm look smaller. In some shots, Arnold's triceps are disproportionate to his biceps and hamstrings looking disproportionately smaller than quads - most notably in the side chest pose. At the 1968 IFBB Mr. Universe, his conditioning was horrible and he could have been alot bigger at the 1980 Mr. Olympia.
Sergio also had his flaws. Disproportionately small biceps, coupled with long biceps insertions and lack of detail between biceps and triceps when viewed from the front and back. Before 1980/1, a lack of detail between the muscles of the upper back, particularly in the back double biceps. Arnold always had great back detail and beats Sergio at the 1972 Mr. Olympia, because of superior detail and posing ability in this pose.
Make no mistake about it, though, Arnold had a very thick back and could match Sergio for thickness when they were competing together in the '70's.