I'm a simple guy, I based my decision off individual subject rankings and things I've heard from past students.
I originally got an offer from USYD to study a B Sci/B Advanced Studies with a double major in Physics and Computer Science, which is exactly the same as the degree I'm studying right now.
However USYD is not ranked as well as UNSW for Computer Science and for Physics both are essentially equal. I had also heard by many people that USYD has outdated facilities for Physics as opposed to UNSW's newer ones. This convinced me to reject USYD and apply for UNSW which I did.
Other small factors that influenced me a little were the fact that many people from USYD said negative things about the community.