A comparative performance analysis of selenium composition variation in Cu2ZnSn(SXSe1-X)4 is analytically performed in this paper. We obtained the optimal stoichiometry (Cu2ZnSnS1.8Se2.2) where efficiency is highest. Comparative simulation brings out the trend of device characteristics variation with stoichiometry. The comparative simulation for band diagram shows the band banding at rear in CZTS whereas it is absent in CZTSe. This highlights the issue of back contact in pure sulphide CZTS. The cut off wavelength increases in QE plot as we move from CZTS to CZTSe compositions. The VOC-illumination and VOC-temperature plots shows (i) VOC saturation in CZTS and (ii) a linear rise of VOC for CZTSe. Nyquist impedance plot shows impedance decrease with selenium substitution. Se substitution enhances the chalcogenide opto-electronic properties as a whole and enhances devices performance. This enables higher efficiency of the CZTSSe devices as compared to the pure sulphide CZTS devices. An optimal stoichiometry obtained is (Cu2ZnSnS1.8Se2.2) with efficiency of 15.3 %, pure selenide CZTSe has efficiency of 13.4 % and 11.6 % for pure sulphide CZTS solar cell.