as for the debate between UoM and Monash, I have had similar indecisiveness trying to find the right place for myself, here's where I ended up:
Monash is closer to home, the campus doesn't have as many beautiful buildings, but it is MUCH more spacious
Monash has more "average" people than the elitist UoM (if you know what I mean... some UoM'ers might argue against that, but this image certianly permeates its prestigous name)
UoM forces you to do breadth, whereas Monash has electives you can choose from anywhere, i.e. you can choose to do breadth only if you want to do breadth.
the double-degree, though extending a normal 3 year course to 4 or 5, offers SO many possibilities, and if your interests are far and wide, I think you will find that a lot more enjoyable
Monash give away more scholarships
Monash freely gives away study-abroad scholarships. Recently I've received so many emails from Monash public announcements that try to get students to exchange to their Sunway (Malaysia) campus, with practically everything paid for (except for your spending money, of course)
if you decided that the double-degree option is not for you, you can always go on to do masters after your 3 year bachelors course (or do honours and go on researching), the same timeframe as UoM, just without the breadth (if you choose not to), and you can do whatever you enjoy best
so that's my 2c.
you might also want to check out the other courses offered by the faculty of health sciences at monash (forgot their full name), from memory most of them allow you to study immunology.