Not quite apples with apples though. Williamson has had the chance to bowl on spin-friendly sub-continent pitches in his test career while Craig has only ever bowled on very unfriendly NZ first-class green tops.
But I agree with the general premise, it would seem to me that Williamson is the 2nd best chucker, oh I mean spinner in the country at the moment.
My feeling is that it's dumb to decide you need an offspinner because the Windies have good lefthanders and turning pitches, and then just pick someone who really doesn't seem good enough because he's the best available (I think Patel made himself unavailable for this tour). Sodhi's most dangerous ball is his googly anyway, so stick with him.