As players, Duncan and KG are equal.
Duncan (career): 20.2ppg, 11.2rpg, 3.1asp, 0.7spg, 2.2bpg
KG (career): 19.1ppg, 10.5rpg, 3.9asp, 1.3spg, 1.5bpg
They're the top-2 active players in Win Shares.
Duncan had the luck of playing on better teams with a better coach (Pop vs. Flip Saunders, not Pop vs. Doc (though that can be argued)) for the better part of a decade. That is reason why he has 4 championships to KG's 1. Duncan never languished in San Antonio the way KG did in Minnesota.
Both affect individual games in a positive way and both have affected the league in a positive way.