It doesn't matter what your second degree is when it comes to being a lawyer because you learn everything you need to know within the law degree itself. I don't know if individual employers would prefer a second degree in science but they are hiring you to be a lawyer not a lab technician. I would suggest you choose a second degree that interests you and that you could potentially use for employment purposes if you decide against practicing law. Having said this, I believe a Science degree is a good way to break up the constant reading and essay writing that paves your path through law school. It's also better than a Medical Science degree because it allows you to choose from a wider selection of electives.
Commerce is generally regarded as a useful degree when combined with law but as I said, everything you need to know for the practice of law is within the law degree regardless of whether it be medical law, torts, criminal law, etc..
edit: I just realised I didn't answer your initial question.
If you know you want law then go to UTS and transfer later because if you get poor grades in 1st year at UNSW you won't be able to internally transfer while if this happens at UTS you're already in a law program so you can fix the situation in 2nd year. (I'm pretty sure I saw someone already say this)