Both unis are decent for mathematics. USYD is good too, while we're at it. Some really important aspects to consider are whether you would prefer trimesters (UNSW) or semesters (USYD/Macquarie), and which uni is most convenient for you (e.g. geographically).
If you plan on going beyond bachelors, it might be too early, but it's worth thinking about what kind of maths you're interested in studying. I'm going to loosely assert that overall, UNSW is more attractive for applied mathematics while USYD is more attractive for pure mathematics. For example, measure theory (a topic in analysis) and Galois theory (a topic in abstract algebra) are offered by USYD as advanced third year units, while UNSW offers them as honours coursework only. USYD also offers some applied topics like control theory and integrable systems in their honours, which I'm not 100% sure if UNSW offers. Meanwhile, UNSW offers a third year cryptography course (which USYD maths is lacking in), and probably better courses on financial/actuarial maths and statistics. [Note: I'm probably biased in favour of USYD, given that I study there and I'm more familiar with the faculty.] With regards to Macquarie, I have no idea. Their site doesn't seem to offer as much information on their units, and I can't find any information regards honours coursework for mathematics.
Actually, now that I've looked up the maths faculties in the three unis, I'd recommend UNSW or USYD if you're going for honours/postgrad mathematics.