I can be honest and say that my hate for Durant makes me less than objective when ranking him amongst the all time greats.
However, I can also make a great argument that peak wise, Dirk is superior. While Durant may have the edge in raw numbers, Dirk has proven himself more in the playoffs. Dirk joins a select few (Kawhi is another player) who elevates his game in the playoffs. Dirk also didn't have the luxury of playing for stacked teams in order to win championships.
In other words, Dirk did more with less than Durant in their peaks (at least so far). I can't really use the fact Dirk beat down the OKC with Durant/Westbrook/Harden/Ibaka because they were all young... but I can use Durant's inability to get the ring without having to join a 70+ win team. To me, this exploits his lack of impact in spite of his highlighted raw and advanced numbers. Furthermore, when we compare their playoff numbers, they are in within the same ballpark. The only difference is, Dirk did more with less as I mentioned before which indicates he had a higher impact on the game than Durant at their respective peaks. Although, Durant is still young enough to do something other worldly... I just think it's highly unlikely he'll top Dirk.
Just imagine Durant having to roll with Erick Dampier and Josh Howard. He wouldn't even sniff the finals with that #2 and starting C.