By means of the close coupling method we have calculated state-to-state differential and integral cross sections for rotational excitation and inversion of NH3 by collisions with Ar and He. For NH3-Ar we used an empirical and a scaled ab initio potential, for NH3-He an ab initio potential. The differential cross sections for NH3-Ar obtained from the empirical potential have an angular dependence that is in closer agreement with experiment than those obtained from the scaled ab initio potential. The integral cross sections are reproduced equally well by the two potentials. Also for NH3-He the differential cross sections are in accordance with experiment. For the integral cross sections the agreement is good too, except for the very small cross sections to some of the higher rotationally excited states. For both complexes the differential cross sections show a strong dependence on energy, both in their angular dependence and in their relative magnitudes.