A nuclide A undergoes $\alpha$- decay and another nuclide B undergoes $\beta$- decay 

In $\alpha$- decay, the entire energy is carried away by the $\alpha$- particle as its kinetic energy. In $\beta$- decay, the energy is shared between the $\beta$- particle and the anti-neutrino. Hence the speeed of the $\beta$- particle will vary, depending on the energy of the anti-neutrino.