depends on what course your really interested in. commerce or law.... that way if your not happy about the uni, at least you can save time and hecs and trasnfer, to the same course, to the uni you want to go to and get credit for the units you've already done! but if your considering combining both commerce/law, i'd say go to the uni thats closest to home so that you don't have to move out.
oh you should also consider living expenses. i'd be harder to move out on campus away from home to study, financially and emotionally. not that i know cuz i still live at home, but i'm considering trasnferring from usyd to anu/uc and i don't think i'd like to be away from my family and friends!!
either of these uni's are pretty good. i'd say they are pretty much all equal and known for their commerce/law degrees, i can't comment on melb cuz i've never been there!! i'd be happy with a degree from any of these uni's!!