I do Bio as my only science subject. Personally I hated Physics and Chemistry in my junior years, which is why I picked Bio in year 11. Bio is pretty much rote learning and understanding the content. It's not that difficult as compared to Physics and Chemistry.
In regards to scaling, it does a scale a bit lower than Physics and Chemistry but it definitely scales better than some Hizzy and TAS/Home Ec subjects (Business, SAC, Aboriginal Studies, Food Tech, Design and Tech etc).
Ultimately it depends on you though, and what you are interested in and what you will enjoy learning
In regards to scaling, it does a scale a bit lower than Physics and Chemistry but it definitely scales better than some Hizzy and TAS/Home Ec subjects (Business, SAC, Aboriginal Studies, Food Tech, Design and Tech etc).
Ultimately it depends on you though, and what you are interested in and what you will enjoy learning