Ylides nucleophilicity comparison
It might not be the only factor, but I think that the steric effect of the huge phenyl groups in Wittig ylides might contribute in making them less nucleophile.
If you give a fast look at a "space filling" model of the two molecules, you see that the negatively charged carbon in a Wittig Ylide is much more masked by the phenyl groups

Note that in order for a nucleophile-electrophile interaction to occur, an electrophile should approach to the carbanion's HOMO orbital (approximatively orthogonal to the screen in both the pictures).