They both embody two different of kinds of archetypical female, equally fascinating:
Nami is youth, energy, sensuality incarnate. Someone whose vitality is overflowing and overwhelming.
Robin instead is class, mystery and conscius charm. Someone who’s more like an enigma to be grasped and whose attractrivness comes with her worldly experience and great intellect and sensitivity.
I can imagine the first as a juvenile, modern, open minded Scandinavian or North american with an indomable youthful spirit. The second as an alluring, mysterious and classy east/mediterranean european.
I think I’m the kind of person who would go more along with the second, but craves for the first (staying in the realm of the thought experiment, of course).