This paper presents theoretical investigations on the interfacial thermal conductance (Kapitza conductance) in both monotype Si/Ge (cubic 3C) and polytype (cubic 3C/hexagonal 2H) Ge interfaces by using full band extensions of diffusive and acoustic mismatch models. In that aims, phonon dispersions in the full 3D Brillouin zone have been computed via an atomistic adiabatic bond charge model. The effects of crystal orientation are investigated, and the main phonon modes involved in heat transfer are highlighted. According to our calculations, polytype interfaces without any mass mismatch but with a crystallographic phase mismatch exhibit a thermal conductance very close to that of Si/Ge interfaces with a mass mismatch but without any phase mismatch. Besides, the orientations of Ge polytype interface that have been observed experimentally in nanowires, i.e., along /[505⎯⎯1], exhibit the lowest interfacial conductance and thus may offer new opportunities for nanoscale thermoelectric applications.