Why is sulfide a better nucleophile than the structurally equivalent alkoxide?
Well, which is the LARGER anion...?
Certainly, third period sulfide anion is LARGER than second period oxide anion....and on this basis its electron cloud is more diffuse, and more polarizable, and thus more likely to participate in a substitution reaction. And CLEARLY, sulfide is a WEAKER base than hydroxide....and clearly the
Taken together, these factors contribute to the superiority of sulfide as a nucleophile...and the superiority of hydroxide as a base.