Also how would someone compare legal to further? I would be happy to do either but can only do one. Which could I score better in? I know that further is extremely competitive and easy to lose marks
I don't do further (or any maths for that matter...), but I can also confirm that legal is an awesome subject (in my opinion anyway)
If your graduation year is 2018, you might have a different study design to the current one, however there doesn't seem to be any drastic changes (content-wise) with the legal study designs, so the current one should still give you a fair indication of whether you'll enjoy the subject (and it might even be the study design you use).
What other subjects are you planning on doing? Are you doing any other maths? And would you say you're a strong maths student? Based on what I've read and seen, I'd probably say that legal would require more work than further, but that is definitely subjective, and dependent on the individual's strengths. I think that anyone who is interested in legal, and is willing to put in the required effort, definitely has the potential to do well in the subject. In contrast, if you're a strong maths student, you might not have to put in as much effort to get those higher scores in further - however that being said, small errors and mistakes can potentially have a substantial influence, study-score wise, in further, whilst this isn't so much the case in legal.
Ultimately, I'd go for the subject that you think you'll enjoy the most. If I were doing my subject selection again, I'd still (absolutely) pick legal, because I find it really interesting, and I enjoy subjects where you can apply what you learn to normal life. But obviously the decision is yours to make
One more thing - do you have any idea what you might want to do or study post-year 12? Because prerequisites should also be taken into account!