The electron drift mobility ratios in 4H- and 6H-SiC have been calculated accounting for the acoustic phonon and the ionized impurity scattering. A comparison of our theoretical calculations and published experimental data suggest that the acoustic deformation potential tensor of 4H- and 6H-SiC is rather isotropic with a constant E-1 = 10 +/- 0.5 eV for both 4H- and 6H-SiC. A factor, strongly influencing the electron mobility anisotropy is the electron effective mass tensor, which may be rather anisotropic as in case of 6H-SiC.