Hello!
Have you liked your first year subjects in journalism? Let me just start out by saying that journalism is not a very good field to narrow down to right now - the job prospects are relatively dismal and the chances of you getting a steady well paying job for quite a while after you graduate are slim. To be eligible for secure jobs rather than freelance work, you typically need to have a great deal of freelance and voluntary work behind you to back up your CV. So basically, you'll be earning a low income for quite some time. This may not bother you! But since law is the other field you're targeting and that has much more secure and better pay I thought I would point this out.
The Melbourne course is designed to be very theoretical and nonspecific - you can take journalism or PR subjects, as I'm sure you know but you don't really get much practical experience like you would at RMIT. That's a much more hands on course. Additionally, the new major in the Arts course doesn't allow you to do an internship like my course did (even though I was going to take the option) which can put you a little behind straight journalism courses like those offered at RMIT.
Journalism at RMIT is a course for those who are sure they want to do journalism. You take few electives and your course is truly preparing you for a job in the media. There's no reason you couldn't do postgraduate law after doing a Bachelor of Journalism, though.
The attractive thing about sticking with Arts is the manner in which you are able to undertake a double degree and keep more options open by studying more broadly. When I was still in Arts that (for me) was going to be Psychology and Media Comm so I had two doors open at the end (and even more because I would have undertaken postgraduate study).
So, here's what I would do if I were you: stay at Melbourne. A lot of people move into straight journalism from Arts and decide that it's not for them. Then they have to transfer all over again or look for new options. When you're not sure what it is you want, it's best not to narrow down. Keep as many pathways open as possible - especially in this day and age. These days, everyone is a journalist. You don't need a degree, you just need an internet connection and a blog. If you're truly passionate about written communications, there's no reason that you shouldn't be able to do this upon graduating...but narrowing down your course when you're still not completely certain is always dangerous.
Maybe put in a VTAC application now (when it's considerably cheaper) and think about it for a while, and decide about whether you're certain you want to do Journalism. If you're not sure by the end of the summer, then you probably should stay where you are...but who am I to talk. I tossed up between two VTAC offers fifty times in twenty minutes at the beginning of this year and chose one 3 minutes before the offers lapsed. I DON'T RECOMMEND THAT PATHWAY, that's for sure!
Best of luck!
PS: if you're wondering why I'm not doing Media/Comm anymore, it's nothing to do with the Melbourne course. I just changed my mind about where I want to go after uni last year.