I've recently become a convert to Haas Unica over Helvetica. My curiosity as to my preference, led me to this:
Essentially, Haas Unica came about as a result of analysing the original version of Helvetica, its variants (as they were in 1980) and similar faces and seeking to improve them - to produce the ultimate archetypal sans serif face. A single face to unite them all, if you like. Looking at the comparitive settings of both faces at text size shows how subtle the differences are, with a detail closeup first:
Read more and see examples at Ministry of Type.