Both UNSW and USyd will both tell you their Commerce Faculty is better. But keep in mind that the cut-off for UNSW is higher than the one for USyd.
As for the overall UNSW vs Usyd argument, whilst my status as a UNSW student would make me anything but impartial, I would say that you should take a number of things into account when choosing your university. Don't forget to keep in mind non-academic factors such as the amount of time it takes you to get to uni each day (eg. it takes me 1 1/2 hours each way to get to uni and back), where your friends are going, etc.
If you know people at uni doing courses you are interested in then talk to them, that is probably one of the best ways. The Good Uni's Guide is good, but is more of a general overview and not really designed for your undividual needs but rather to give you an overall idea.
And of course, don't forget about UTS, UWS and Macquarie!