In short, the data science major at USYD is a mix of statistics and computer science, whereas the Data Science and Decisions course at UNSW in addition has compulsory economics in it.
In my honest opinion (as someone who has studied statistics, computer science and economics), economics is worthless for data science. You'd be better off spending those extra credit points on more stats/comp sci/maths units, or anything you personally find interesting. Apart from that, doing data science at either university is just as good.