Hmm this is an interesting question. They both score in unique ways.
Offensively Harden does more than KD, because not only is he a great scorer but he’s clearly a better playmaker and passer than KD is. Harden legit has PG skills. KD is a pure wing player.
What makes Harden unique is that he scores so much due to his ability to draw fouls and his tricky ability to goad the defender into a foul. He also takes a lot of step back 3s and is good at those. He’s like some kind of new-age Adrian Dantley in that sense, but also has more range than Dantley ever had.
Kevin Durant is also unique, he’s 7ft tall but instead of playing in the interior like most 7ft guys did in the past, he plays on the perimeter and shoots better than most guards do. That makes him essentially a more athletic version of Dirk Nowitzki (a player who is already very difficult to guard).
So who’s more difficult to stop.. Adrian Dantley with more range, or a more athletic Dirk? It’s tough but.. I’m going to say that Harden is more difficult to stop in the regular season but he tends to slow down and shrink a bit in the playoffs but KD is just as difficult to stop in the playoffs as he is in the regular season. In the playoffs, KD all day every day, so I’ll probably pick him overall.