I don't know a lot about Monash, but I think making a generalization about Monash students having less motivation is pretty poor.
Anyway, if you have an offer then I wouldn't throw it away because there will be thousands of VTAC applicants who will have no offers, So congratulations. As for fearing your knowledge won't be up to date in chemistry and physics, take note that there is introductory subjects for both.
In the end if you want to do Zoology at Monash and what is pulling you toward Melbourne is it's prestige, then maybe you may want go with your gut feeling. In the end both campuses are excellent so keep that in mind too.
I just want to also say to everyone that Melbourne is starting to make changes to the breadth in the BSc, in that you only need to do 4 breadth subjects.