Two systems of noncentrosymmetric cubic helical magnets Mn_{1 - y}Fe_ySi (y = 0.06, 0.08, 0.10) and Fe_{1 - x}Co_xSi (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.50) have been compared. The concentration dependences of the critical temperature and magnetic field have been obtained using small-angle polarized-neutron scattering and analyzed in the framework of the Bak-Jensen model. It has been established that, among the two interactions that play the main role in these systems, i.e., the isotropic symmetric ferromagnetic exchange and the Dzyaloshinskii-Moriya isotropic antisymmetric interaction, the former interaction determines the critical temperature in the Mn_{1 - y}Fe_ySi system and the latter interaction determines this temperature in the Fe_{1 - x}Co_xSi system.