Shaq, imo, is way overrated. Yes, he had the potential to be the greatest of all time, but he wasn't. How can the most dominant big man lead the league in ppg once in his career? No rebounding les, no blocked shot les? He devoured some teams in the playoffs, but it was strictly due to matchups. Shaq has always struggled vs. the Spurs (ppg way down), and made his name playing teams like the Pacers, 6ers and Nets in the finals.
People tend to forget the fact that when Shaq was at his prime along with Hakeem and Robinson, he was clearly a notch below the two, it wasn't till Hakeem and Robinson both got old that Shaq had his share. And Shaq has always have a very good sidekick. Sure you can make the argument that he makes his teammates better, but the year after Shaq left, Penny Hardaway was still dominating the league until he suffered injury after injury to his knees. Kobe did fine last year, and Wade was on the verge of s om in last year's playoffs anyways.
Duncan, on the other hand, carried his team time and time again, and lead them to 3 championships (same as Shaq) with no significant offensive help in two of them. Shaq was magnificent in 2000 and 01, but that was it, he had a higher peak, but the peak was very short due to his reluctance to take care of his own body. Duncan, on the other hand, had been consistently excellent throughout his career, and is more like a plateau.