Physics scales higher than biology, but that's not to say you'll automatically do better in phys than bio. Bio can scale well if you do well. There's a couple of articles on scaling - I think Matrix and Talent 100 have "HSC Scaling explainers" for you to read.
I do not do biology (I do phys though...haha), but I can tell you what I know about biology.
Biology (from what my friends have told me) is really content-heavy and there is a lot of information to absorb. They also tell me it is hard to concentrate in class- but it could just be them, or their teachers.
If you are good at absorbing content - then biology is good for you! If not...well there are plenty of easy-to-read notes on this website for you to browse through.
Oh, that reminds me! Why don't you look at some sample papers and some biology notes from this website. Or ask around, maybe. You can gain a better understanding of biology from that instead of my little ramble.
Hope this helps!