Phase change materials including GeSbTe and AgInSbTe have successfully demonstrated multilevel switching capabilities, yet achieving precise controllability and reproducibility are crucial towards technological applications. In this study, we demonstrate nine distinct optical levels in Ge2Sb2Te5 (GST225) and Ag5In5Sb60Te30 (AIST) phase-change materials using pump-probe experiments under identical device conditions and the role of the crystallization mechanism is examined for realization of reliable multi-level programming. Nucleation-dominated GST225 material corroborates improved performance characteristics of low threshold fluence (6 mJ cm-2), least optical variation (±0.25%), and high reflectivity contrast (∼2.5%) between any two consecutive levels as compared to growth-dominated AIST material. Furthermore, the opto-thermal simulations depict a gradual change in the crystalline fraction in GST225 and an abrupt change in AIST, which further confirms the improved controllability in nucleation-dominated crystallization. Hence, these identical measurements along with the opto-thermal simulations elucidate that the role and nature of crystallization play a critical role in precise control of variation of reflectivity in multi-level states of GST225 and AIST, respectively. These findings will be useful towards the development of reliable multi-bit phase-change photonic memory devices. © 2020 IOP Publishing Ltd.