I will preface my statement by saying that i am applying for university this year and thus have no first hand experience of either university.
However I have tried to do some research comparing both universities. It seems as though USYD wins for law (prestige, old school teaching) but UNSW wins for commerce but by a much greater margin. UNSW i have heard has a new structure for comm/law enabling smaller class sizes, an advantage over USYD.
However when deciding between both they are so close it will not make much of a difference either way and will come down to personal preference i believe.
But hey that is just my two cents worth...