I guess it really depends on where your interests are... which I know isn't very useful. If you know that you aren't going to do law in the long run, or if you think you need a little more time to decide, a single degree is probably a better option, especially when you consider that you can concentrate on your scientific/intellectual interests before doing a JD (and its not just the Melbourne JD out there, many places offer it). However, if being a lawyer is something you want to do, or if you know you would like to work in a law-related area at some point, you should do it as an undergrad.
I considered Law. I didn't do it because I wasn't ready to commit to that many years of study, especially given that I wasn't sure I wanted to be a lawyer and especially given that I was given the opportunity to focus on subjects I knew I was interested in. It was always just a possibility to me, rather than something I felt I should do. But, if you can commit to it, and if you think you won't be able to handle full-time Science or whatever, it could be really helpful to you. Remembering, it's also possible to transfer out of Law too!
I know it's a hard choice! My favourite way out of making said choice is following the pretty scholarships (in which case, you're more likely to get one for not-Law), but I don't recommend this highly. Maybe if you know someone who is relatively unbiased and a lawyer, have a talk to them? Or a careers adviser? Also, remember that if you work either as a lawyer or a scientist, it'll be for over 40 hours a week for years of your life - which sounds less like something that would trigger some kind of descent into madness for you?