Duncan had better longevity (partly not Robinson's fault due to his Navy commitment) and more accolades, but Robinson at the peak of his prime was a more elite player than Duncan per whatever advanced stat you like.
Duncan's highest ever VORP year was 8.0 in 2002. Robinson had three different seasons better than that, including 11.4 in 1994.
Duncan's highest ever PER season was 27.1 in 2004. Robinson had six different seasons better than that, including 30.7 in 1994.
Duncan's highest ever BPM season was 8.5 in 2004. Robinson had six different seasons better than that, including 11.9 in 1994.
Duncan's highest ever WS season was 17.8 in 2002. Robinson had two different seasons better than that, including 20 in 1994.