With good grades you'll walk away from any of those uni's with the same opportunities.
Usyd has just updated/revitalized their course for those commencing 2012, also they are now one of the only uni's I know of where it's possible (provided you have the undergrad grades to qualify) to receive a CSP, effectively reducing your student-fee loan by a third.
UNSW seems to have a wider and more inviting array of courses judging from their handbook, however they make you take 4 very general/introl level courses in your first semester that sound very 'soft' and are probably just about learning buzz words and giving presentations; this also limits you to only being able to do 1 major/specialization within the standard 1.5 year time frame, whereas almost every other uni allows you to do 2.
ANU has the best global ranking if that means anything to you, I haven't done the research to really comment on their course structure or contents.
Macq has a good reputation, and great industry contacts that they brag being able to place you with during your MCom degree such that you walk away with some experience, which could be a major plus. Also they are slightly cheaper by a few grand in terms of total course cost (not so much their MComm, but if MProfAcc interests you as well, then theres is $27k as opposed to ~$33-35k at other uni's)
Can't comment on UMel.