The influence of the cluster-core unit in cluster-decorated -Si on photo-electrochemical (PEC) hydrogen evolution has been investigated using a homologous series of cubane-like heterobimetallic sulfide compounds. These compounds stem from the generic cluster structure or (, Mo; , Cu). We find that the Mo-based () cluster-decorated Si photoelectrodes show higher PEC performance than otherwise equivalent W-based () cluster-decorated ones. This is consistent with higher electrocatalytic activity of the Mo-based clusters supported on -Si when measured in the dark. The result of stability tests is that photoelectrodes decorated with clusters without Co () can exhibit promising stability, whereas clusters of the structure (, Mo) yield photoelectrodes that are highly unstable upon illumination. X-ray photoelectron spectroscopy (XPS) results suggest that both oxidation and material loss play a role in deactivation of the materials. Additionally, we observe that the photocurrent depends linearly on the light intensity in the limiting current region, and the corresponding incident photon to current efficiency (IPCE) may reach approximately 80%. Density functional theory (DFT) calculations of the clusters adsorbed on the hydrogen-terminated Si surface are used to estimate and compare cluster adsorption energies on the surface as well as the H-binding energies, which is a descriptor for electrocatalytic activity.