First of all, there is a course information day at Melbourne in a few days, I'm not sure if Monash do the same thing. Heres a link:
Course Information Day - Future students
I don't know much about Monash's courses so I can't really recommend one over the other, hopefully someone else will provide information for you for Monash,
but I just want to bring to your attention that you can also do the Bachelor of Science at Melbourne and major in one of many avaliable engineering majors, as opposed to doing the old Bachelor of Engineering. Heres some sites for you
Science Majors - new generation BSc : Science Student Centre : The University of Melbourne
Undergraduate Studies in Engineering: Melbourne School of Engineering : The University of Melbourne
Bachelor of Engineering: Melbourne School of Engineering: The University of Melbourne
Engineering within a Bachelor of Science: Melbourne School of Engineering: The University of Melbourne
Its basically the same as doing the BEng I think, although you might want to look at both course structures on the internet if you want more information.
At Melbourne a quarter of your subjects need to come from another faculty, in what is called the breadth componant of your degree. This can be a very good thing if you have a wide range of interests. You can generally just pick subjects that interest you. I'm doing mainly philosophy in my breadth for example
Melbourne is generally ranked well over Monash as a general university, usually in the top 1 2 or 3 in Australia, but this should only be a slight factor in your decision. Both are good Universities.