I personally agree that Westbrook is better than Harden, but I don't think the gulf is as large as you seem to be suggesting. Harden's much more poised, a better shooter, more efficient at scoring, better at controlling pace, and slightly better at not turning the ball over for his career (though since being made a full-time starter, he's roughly on par with Westbrook for TOPG). Westbrook's obviously far more athletic, much quicker, and more effective at breaking down the defenses just because he's so explosive.
I give Westbrook the edge primarily because of his energy and effort: he rebounds like crazy, he can play some extremely intense D when focused, and no one can ever accuse him of not leaving it all on the court when he's playing. Harden, on the other hand, has been the worst starter on the defensive end throughout these playoffs so far, not to even talk about how much of a sieve he's been during the regular season. While Harden can bail out a team with timely scoring, I don't think that's quite as necessary on a team featuring the best scorer in the league in Durant. I find Westbrook's passion to be much more complementary for a team that often goes through emotional lulls.