Rarefied gas flows obey to the Boltzmann equation, but numerical simulations of this equation are not always possible, so that simpler models have been introduced. The ES-BGK equation is one of these models. It gives the correct transport coefficients for the Navier Stokes approximation, so that Boltzmann or ES-BGK simulations are expected to give the same results for dense gases, but in the case of a rarefied flow, complete numerical comparisons are needed. In this paper we present numerical comparisons between the two models in transitional regimes (where the ES-BGK model is expected to be useful) for reentry flows around a compression ramp and a plate. We also emphasize that the ES-BGK model gives flow predictions closer to the Boltzmann result than the simpler BGK model.