Uranus is the preferred flux density calibrator for the ATCA. Mars is less suitable for a number of reasons: some models of Mars' average brightness temperature suggest variations in the temperature of up to 10% as a result of Mars' diurnal rotation, its distance from the Sun and the viewing geometry of Earth-based observers. Definitive measurements and confirmation of these models are poorly studied in the literature. Depending on its distance and the array configuration, structure on Mars is also readily detected with the ATCA: Mars has hot equatorial and cold polar regions.
Although Uranus or Mars (preferably Uranus) are the best flux density calibrators, frequently they will not be accessible to an observation. To account for this, the Narrabri Observatory regularly monitors some additional sources (usually 1253-055 and 1921-293) to allow them to be used as flux density calibrators. Be warned though that these sources can vary substantially on month timescales. If Uranus and Mars are not accessible to your observation, you should enquire about the next best source to be used for flux density bootstrapping.