Under the generic term Stranski-Krastanov are grouped two dierent growth mechanisms of SiGe quantum dots. They result from a self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. We elucidate here the puzzling dierence between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the nucleation time scale and ATG characteristic time, we show that the former exhibits a strong exponential divergence at low strain while the latter behaves only algebraically. Consequently, at high/low strain, nucleation/instability occurs faster and inhibits the alternate evolution. The cross-over between the nu-cleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50%