people often choose senior science if they want to do teaching or they cant handle the specific sciences such as chem, bio or phys so they do a simple mixture of the core sciences = senior science.
Don't choose one or the other because they're easy, choose them for their course content. Senior Science is basically just year 7-10 science and a little bit more, so it's piss easy, and scales shit as a result. Biology is obviously totally different.
So it depends on what you want to do after school/what you're more interested in.
I actually find bio easier because all of the topics are interrelated rather than senior science where you do a little bit of everything such as waves, the heart etc...
i guess its rlly just up to you what you would be able to perform best in and the one you enjoy