OP you should ask Russ
What do you think I'll say lol?
In terms of wanting to do medicine, biology is far and away more relevant. Anything relevant from psych is just a subset of biology (heck most of psychology is a subset of biology). That said, you definitely do not need to study biology at VCE to get into medicine or to do well in medicine.
The new course design for Psych is quite interesting (what I've seen of it at least) and is both interesting and broad. Biology will be harder and probably lacks the breadth of the psych course but has the major selling point of being fundamentally related to every day life. When you study genetic mutation, you'll get a new appreciation of what cancer is etc. Psychology can give you the same feeling, but it's usually in the areas of study that are just taken from the biology course.