The effect of the Ge and C in a-Si1-xGex:H and a-Si1-xCx:H thin films on the optical gap and valence and conductivity band tails (VBT and CBT) from one side and on the short-range order (SRO) from the other, have been studied by optical absorption, photothermal deflection spectroscopy and Raman scattering measurements. The results show that the VET is very sensitive to the geometric disorder as a consequence of the incorporation of Ge, whereas the CBT is not. CBT increases with Ge concentration when x > 0.33 and this increase is due to the alloying rather than to the short-order deterioration. Both the CBT and VET, as well as the short-range order deterioration increases with the C content in a-Si1-xCx:H films and the effect of C incorporation on these values is stronger than that of Ge in a-Si1-xGex:H films. Probably this is due to the greater difference in electronegativity between silicon and carbon than between silicon and germanium. A comparison of the correlation between Raman Si-Si TO-like peak width and the optical gap for films with different Ge and C concentrations allows to separate the roles of Ge- and C-induced bond angle disordering and alloy energy band changes.