Electronic and optical response of two ternary isostructural phase change materials PbSb2Te4 and SnSb2Te4 are computed using the full potential linearized augmented plane wave scheme. Accuracy of electronic and optical properties was ensured by employing the modified Becke-Johnson (mBJ) exchange potential within the density functional theory. The energy bands and density of states show a band gap of 0.48 and 0.30 eV for PbSb2Te4 and SnSb2Te4, respectively. First-ever optical features, such as dielectric constants, refractive indices, reflection spectra, extinction coefficients of PbSb2Te4 and SnSb2Te4, are calculated and analyzed to explore the applicability of both the compounds in memory storage devices.