We present a comparative study of GaAs nanowire growth on Si(111) substrates by molecular beam epitaxy with the assist of Au and Ag colloidal nanoparticles. Our approach allows the synthesis of nanowires with different catalyst materials in separate sectors of the same substrate within the same epitaxial process. We match the experimental results to the modeling of chemical potentials and nanowire length distributions to analyze the impact of silicon atom incorporation to the catalyst droplets on the growth rates and size homogeneity in ensembles of Au and Ag-catalyzed GaAs nanowires.