have you been to their open days or other events? They can help you get a feeling for the campus and if you enjoy it there.
Both USYD and UNSW have great reputations for science and are high in the rankings. Though don't put too much stress on rankings, they aren't always a great representation of the uni as a whole. In saying that, both unis are well-regarded and you will most likely get a high-quality education from both.
Do you have a very specific career or area of interest? If so, look into each unis research in that area; one may have lots of interest/academics that specialise in it. If not, don't worry too much about this. Both will have opportunities and if you have something specific you want to do later, and you can easily move if needed or change unis for honours/post grad
Don't underestimate the value in which uni seems more enjoyable for you. We think so much about education, marks, employability etc, and forget how important it is to enjoy life. If you love uni and life in general, you are much more likely to study and succeed, in turn making you a more employable, high achieving graduate also. Whereas if you hate your uni, you aren't going to study and will fall behind and make everything harder for yourself. For example, imo the usyd campus is MUCH nicer that unsw, its closer/easier for me to get to, and its closer to the rest of the city. I'm going to usyd partially for these small reasons, which add up to a lot more. If you enjoy being on campus and enjoy learning, you will do better. For some, unsw might be more convenient, or there might be other specific factor/groups/events/programs that entice them there. So think about your uni student experience too.
Remember you can get multiple uac offers (as long as you change your preferences), so you can give yourself both options.
As to the advanced science, i would say do it if you can. You can easily drop to the standard b.science if needed, without repercussion. You just continue you're degree, though often with some changes as to advanced units later on. My understanding is that you get different prac classes to the rest and some other advanced units throughout the degree. No harm in at least trying it, but not a huge deal if you don't get in.
Finally, don't stress too much. You have two great options, and things can change later on if needed - uni is a lot more open and flexible than most people seem to think. Good luck! Also have a look at the uni websites, there might be some more events in jan if you want to have a look around each uni
