We present an experimental analysis of strain relaxation in SiGe/Si heterostructures which is strongly dependent on the Si substrate orientation. In particular, different driving forces for the transition from 2D towards 3D growth were evidenced. On (100) Si substrates the primary driving force for SiGe islanding is the ability to elastically relax strain on the top of the islands. At low elastic energy a small corrugation is initiated at the free surface of the SiGe overlayer which is known to lower the energy of the system. The amplitude of this corrugation increases with the accumulated elastic energy up to the formation of dislocation-free islands, The latter kinetically evolve towards plastically relaxed islands. In the same experimental conditions, the strain is at once relaxed by nucleation of dislocations on (Ill) Si. Dislocation-free islands were not observed in the latter case. Continuous dislocated films are formed up to compressive stresses of about - 0.17 GPa (equivalent to a misfit between the SiGe layer and the Si substrate of approximate to 3%), At higher misfit energy, islands are initiated by the ability of introducing stacking faults on the edges of the islands. The study of defect nucleation processes does not permit one to explain the nucleation of misfit dislocations at lower stresses on (111) Si than on (100) Si. Based on the existing models we suggest two possible mechanisms able to explain the observed discrepancies.