Arial just looks clunky compared to it. Helvetica's loops are more rounded, they stand out more boldly from the staffs than Arial's. It's a much more confident typeface. Arial's 'e' looks squashed, it reminds me of a trollface. Its 's' has the same problem, though not as pronounced. Helvetica's versions are much more elegant. Helvetica's numerals are far nicer than Arial's, in particular the '1' and the '2'. The tails of the '6' and '9' daringly overhang the loop slightly in Helvetica, no such eccentricities in Arial, except for the 'f', whose top pokes out past the cross. It would ordinarily be a fine quirk, but in Arial it just looks out-of-place.