I'm just going to post this here for convenience.
First of all, if you want to do MD at Melbourne, you have to sit the GAMSAT, have a competitive GPA, and also pass the MMIs.
If you're 100% set on doing a health-science, do Biomed. The competitive cohort should motivate you to work harder, it'll essentially be like going to another selective-school cohort (and since you went to MHS, we can both see how that environment helped us).
If you also have other career aspirations/interests that aren't limited to health care, do Science, because it's more flexible and you may end up liking a career that's not medicine. However, if you go into Science wanting to do medicine, chances are, you'll be tailoring your course to match the Biomedicine course but you'll lack the cohort value.