Any advantage Duncan had on Shaq defensively hardly counters the ENORMOUS differences offensively. The thing about Duncan's best year (25ppg) was that it was an anomaly. The best he ever did besides that year was 23 a game.
Lets not forget the league INVENTED A RULE to stop teams from Hack-a-Shaq-ing in the final two minutes. Teams decided their only defense against Shaq was to foul at the start of every possession.
Duncan wasnt a good free throw shooter either (one year he averaged .599). Teams didnt need to foul because he could be guarded by individual player. You didnt need to send three guys to try and stop him if he got down low.
Shaq was just a different breed and its comical to watch Spurs fans desperately try to make a case for why Duncan was "better". Shaq was the most dominant player in probably any sport outside of maybe Barry Bonds.