Is it fair to say that biology is subjective subject?
I don't think any subjects are subjective. In all subjects, you have a certain set of criteria which you have to fulfil. If you fulfil all of them, you will get all of the required marks. There are always some little grey areas in all subjects, even mathematics.
If you look at the statistics though, and one of my teachers last year was an examiner for English, also friends with the Chief Examiner, and he basically says that within VCAA, it's actually quite well known that mathematics subjects (which are meant to be black and white) actually have more deviation (two examiners giving a different score for an entire paper) than the English exam.
Honesty, the workload depends on how much you want to work and how much you enjoy the subject. No-one can give you a definite answer, both require work.
Also your capacity, some people will just do better in biology, some in physics