Yeah you can take higher subjects regardless whether you are in adv sci or not. But really adv sci at USYD seems like a more attractive option than at UNSW. Not because it has a higher UAI or such, but because generally they have more special subjects there to make you feel special. Just go read the USYD sub-forums, adv sci there really has a buzz there unlike here. I'm not being biased or anything, I do adv sci at UNSW. I think it's good for a math and finance major, since UNSW offers a project for financial engineering at the end where you get to research and write small thesis.
And I think more demand will generally indicate that a course is superior, most people are rational. So the higher UAI at USYD should be an indication of it's superiority or at least quality of students, of course unless someone can prove to me that there are more spots at UNSW for adv sci.
But in the end it's up to you, I chosed UNSW because it had a better plan for math and finance, you should do some research and find out about the plans of each degree, and then consider which one suit you best.