  We explore similarities and differences between widely-used threshold
resummation formalisms, employing electroweak boson production as an
instructive example. Resummations based on both full QCD and soft-collinear
effective theory (SCET) share common underlying factorizations and resulting
evolution equations. The formalisms differ primarily in their choices of
boundary conditions for evolution, in moment space for many treatments based on
full QCD, and in momentum space for treatments based on soft-collinear
effective theory. At the level of factorized hadronic cross sections, these
choices lead to quite different expressions. Nevertheless, we can identify a
natural expansion for parton luminosity functions, in which SCET and full QCD
resummations agree for the first term, and for which subsequent terms provide
differences that are small in most cases. We also clarify the roles of the
non-leading resummation constants in the two formalisms, and observe a
relationship of the QCD resummation function $D(\alpha_s)$ to the web
expansion.
