"Russell" Westbrook "might" be a better player but I'm not convinced he's a better fit for this OKC team. I remember when the team had harden and was it "Eric Minor?" - these two guys could come off the bench and create for other players - a skill that neither Durant or Westbrook are terrific at.
I think Harden and Durant on the wings with Jackson, or any other good point guard distributing would just be so difficult. Westbrook strikes me primarily as a 1on 1 player who doesn't make players around him that much better. Harden's play making ability from the two spot, in addition to his scoring capability is the game changer for me. His defense may not be what Westbrook's is, but Sefalosha can assist there.