I do biology, and while there is a lot of content it's a lot easier to understand than physics (well, depending on what you're better at - more maths inclined people should go for phys/chem, some physics formulas occur in 2u/3u maths)..
Also I really strongly recommend going for what you're better at - do NOT go for a subject because of the scaling. If you end up being crap at it, your ATAR will suffer anyway. Best to do well at biology than bad at physics - scaling won't help you if you fail.
Ultimately it's up to you, ask your teachers about the content, ask them whether they think you should do it (take their advice with a pinch of salt - in my experience some teachers will tell you you're really good at a subject in order to get you to take their subject, so they'll get the numbers), see what teacher(s) will be teaching it (that can affect how well you do - if you don't like the teacher you probably won't want to pay attention in class) and check out some of the notes on the subject.