You will need to major in accounting. Don't worry - you only have to make this decision for 2nd year.
Monash Uni Comm/Law:
+ CSP Place
+ 5 years
+ Won't have the stress of sitting the LSAT
- Less 'prestigious' (highly debatable)
- Teaching quality for comm is slightly lower than Melb. I've talked to my Melbourne Comm friends about this - they do teach better @ melb for accounting.
Melbourne Comm + JD
+ More prestigious (again, highly debatable)
+ Amazing location in the city, easy to get to by train
+ More flexibility to experiment and do breadth subjects and do a double major (you can't double major with Monash Comm/Law), so you can major in, for example, both accounting and finance
+ More time to decide if you actually want to do law
- Extra pressure and uncertainty of sitting LSAT
- More debt to repay: JD is postgrad, costs alot (!) more than undergrad law
- Takes 6 years, whereas Monash takes 5 for the entire comm/law
basically that's what I know so far. I would have gone to melbourne tho, if I made the 99.90 cut off for a full scholarship and guaranteed place in Melbourne's JD

You'll find lots of differing opinions, but generally melbourne takes longer, and is more expensive for law.
They are both great!
good luck

edit: it is actually very competitive to get into the Melbourne JD - lots of people wanting to get in and LSAT can be unpredictable