Matters what PG they could have gotten back. If not, Eric Maynor and Harden starting wouldn't be too grand.
Harden still hasn't been able to show all he's got in that 6th man role in OKC. He's still growing, and he's already a better pg than Westbrook, when it comes to being a floor general.
He may be a better scorer, or more of a "franchise player," you never know. But at this point, Westbrook was already signed, and he didn't stink it up in the finals, at least as much as Harden.
Biggest factor, though, is the need for an athletic, all star pg in the league as of now.