I'm just a VCE kid so I'm not qualified to speak about university experience at length, but I'll have a go at answering your questions:
1.) First off I'd just like to know whether more people prefer doing engineering at Melbourne Uni through Bachelor of Science or Commerce and why?
This depends on the person. I assume you'd do either a b.com with engineering electives or a b.sci (major in eng) with commerce electives for the first 3 years, before you graduate and move on to your masters in engineering. That being the case, it really depends on what degree you want and what subjects you are most interested in, like from the subject map things ive seen you'd do 60% science and 40% commerce in those three years in the science pathway, or you could opt for 40% eng and 60% commerce through the commerce degree. Again, not qualified to speak on this with any authority.
2.) Secondly, what is the difference between, in terms of employment, of doing the double degree at RMIT, and doing the Bachelor of Commerce and masters at Melb Uni?
You can't work as a professional, accredited engineer without the full 5 years (stopping after year 3 with your b.sci or b.com) so you wont be able to get an engineering job until the end. That said, you end up with a master's degree which is a slightly higher qualification. In fact the double degree at RMIT probably goes for 5 years too, meaning you will end up taking the same amount of time to be able to get a graduate engineering job. The difference, then, comes up in the next point;
3.) Why do some people not like the Melbourne Model for engineering?
You aren't guaranteed a place in the master's degree after you graduate your bachelors in the melbourne model. it's subject to your performance. This shouldnt be a problem if you put in the effort, i cant imagine them turning harworking students away, but if a student was to fall into the relaxed uni lifestyle i keep hearing about and ignore their degree, just passing subjects to get through it, they'd probably be turned away from the masters and end up stuck without an engineering accreditation. This is one of the reasons i gather people dont like melbourne's model. It does have its advantages, however, since you have 3 years of a not-so-specific bachelors degree where you can experiment with electives and find where your true interests lie before you get into a specific engineering field. If you dont know what type of engineer you want to be (like me) this is invaluable. With RMIT's courses, as I understand it, you have to specialise from semester 1 tbh, enrolling straight up in the degree of your choice. If you ask me that's kinda chucking yourself into a dark room without a door, you can turn on the light but you dont know if youll like what you see. RMIT would probably let you swap into a related field (eg between aero and mechanical if you were quick about it) but you may have to complete extra units to catch up. What im trying to say is, melbourne gives you more space to get a feel for uni and for engineering, while RMIT is kinda forcing you to speed up your decision.
4.) Which Uni's out of RMIT and Melbourne are regarded as better for engineering?
I hear everyone say 'Melbourne is one of the best ranked universities in the world for engineering, and the best in australia' and 'RMIT is great because it's far more practical than melbourne/monash, so employers want that in their graduates'
and honestly, while those opinions may be fact, thinking of what an employer is going to look at on my resume doesnt help me decide anything at all. If you're a hands on learner, my understanding is that RMIT is better for that, as well as for getting industry experience (through all engineering accreditations require like 12 weeks or something anyway) during your degree. On the other hand, if a focus on high level theoretical understanding is your favourite way to learn, go with melb, because maybe uni should be about learning more than pleasing a prospective employer with a certificate. On that note, 'master of engineering' has a
very nice ring to it
I personally felt like Melbourne Uni had a better feel as a university and just looked like what I always imagined uni to be like. (Doesn't mean I will go there as many people have told me engineering is better at RMIT), where as RMIT was just buildings and buildings. However, I have still not made up my mind and any opinion provided will be appreciated.
I went to the RMIT open day (and to melb/monash) and I honestly just didnt like the feel of the place. The buildings were beautiful and the people were friendly, but there was something about it that didnt sit right with me. I have since ceased to consider it as a preference, and the atar reqs are so close to melb/monash that its not like it was going to be a great backup anyway. My point is, go with your gut, because you're gonna be spending so much time at whatever place you choose so you had better like it.
Good luck, too
EDIT: typos and stuff