A comparative study of simulated air shower longitudinal profiles is presented. An appropriate thinning level for the calculations is first determined empirically. High statistics results are then provided, over a wide energy range (10 14.0-10 20.5 eV), for proton and iron primaries, using four combinations of the MOCCA and CORSIKA program frameworks, and the SIBYLL and QGSjet high energy hadronic interaction models. These results are compared to existing experimental data. The way in which the first interaction controls Xmax is investigated, as is the distribution of Xmax.