I just finished my first year of this degree. The main difference is the advanced subjects which are a requirement of this degree.
Advanced subjects differ sometimes in terms of theoretical content, practical requirements or assessment. For example, in my chemistry course, the difference was in the practicals, which were individual (as opposed to group work) and MUCH more interesting.
Advanced physics had the same general theme as the other physics courses however the material we covered was much harder and mostly different. The prac course was quite simiilar between courses. The practical experiments were sometimes different but the assessment was the same for the labs.
In biology, generally the content and the pracs were the same, but advanced students had extra/different assessments and the labs had extra material on top of the regular material.
Maths courses have overlap between the different difficulty levels. The assessments are not the same but you leave having covered similar concepts compared to the regular people. The regular and fundamentals courses are much more theoretical and you get to see much more about 'where things come from'.
I can't say much about any of the other subject areas because I didn't do them, however those are some examples of what makes advanced science different from regular. On top of that is the TSP (whilst they say 99 I know many people who got <99 and were invited). It is a very rewarding experience; I did a group project and made quite a few friends. You can also do individual projects but I'm not doing those because I have to work.
You also need to obtain a credit average. At USyd raw marks are scaled to follow guidelines which specify how many students are allowed to get Credits/Distinctions/High Distinctions. From what I saw in first year, some departments didn't do this for specific subjects, but for their whole subject area. This meant that advanced subjects had greater proportions of C/D/HD grades than the regular version. Don't take this for granted though; you still need to work.
I would say do advanced science. If it's too hard, then just transfer streams to regular science when you feel like it. Honestly, I think you should be challenged in what you do. It's character building and it prepares you for your career when you might be faced with problems that you need to solve on your own. Considering you're going to do vet science I'd say that being challenged is important.
If you have any more questions I'd be happy to answer them