one difference would be maturity.
the other purity.
the smaller trichomes on any given strain are generally the immature ones. so their cannabinoid profile will be different (higher in CBG).
there are also a couple or three different kinds of trichomes (Multi-photon microscopic images of various types of cannabis trichomes), with associated size and cannabinoid profile differences.
So the concept of sieving for effect then extracting is not unreasonable.