I tend to think that Westbrook is just slightly a better player than Harden, but that Westbrook's "cache" is more better than Harden's, such that "Westbrook + Harden's-trade-return" is not as good as "Harden + Westbrook's-trade-return." Not sure if that makes sense.
Let's try this analogy: Say that as a player, Westbrook is a $.95 player and Harden is a %.92 player, but Westbrook around the league could fetch $.98 value and Harden could fetch $.88 value.
So Harden + Westrook is like $1.87. Harden + Westbrook's value is $1.90 and but Westbrook + Harden's return (Martin, Lamb, couple of variably protected 1sts) is like $1.83.
This may be completely wrong, but I could imagine a scenario where, despite Westbrook being better than Harden, OKC would be better by trading Westbrook instead of Harden because the proportional return would be greater due the reputation and recognition.
We know Harden fetched Martin, Lamb, and 2 1sts of variable protection.
What would westbrook have fetched this summer? (post his salary/pre recent extensions).
Would Detroit have done Stuckey/Monroe/knight or other?
Would Philly have done Jrue Holiday/Thadeus Young/Moultrie?
Would Toronto have done Lowry/Calderon/Valencuinas for Westbrook/Perk?
Would Memphis do Conley/Wroten/Gasol for Westbrook/Perkins?
Rubio/Pekovic/Ridnour from Minny?
Basically, we know that Westbrook+Harden+Durant+Ibaka+Collison/perkins was a title contender.
So accepting that they had to keep only 2 of Westbrook-Harden-Durant, I think there were ways they could have turned Westbrook into a PG that can shoot, share, defend as well as a legit center for greater roster balance and maybe at the same time ditch the Perk contract.