Density functional theory in combination with the nonequilibrium Green's function formalism is used to study the electronic transport properties of MAPbI(3), MASnI(3) and mixed-MAPb(0.5)Sn(0.5)I(3) perovskites (MA-methylammonium). The largest electronic transport is obtained for tin-halide sample due to the delocalization of electronic states in the system. The mixed sample also shows improved transport properties as compared to the lead-halide system. In addition, tin-based perovskites are less sensitive to the spin-orbit interactions, whereas the electronic transport properties of MAPbI(3) are strongly affected by the relativistic effects. These findings indicate the possibility of enhancing charge carrier transport in organometallic perovskites by metal atom mixing.