Arial's 'e' always bugged me. Helvetica has a taller x-height that gives the 'e' room to breathe. Arial's looks like a trollface. Arial's 'a' is reminiscent of an awkward twelve-year old.

Helvetica's shoulder curves have elegance, its 'm' is downright stately. The swoop of Arial's 'g' is timid and fearful. The bowl of Arial's 'o' looks like a stop sign, Helvetica's is properly elliptical.