I'll play devil's advocate and say that there ARE people who do Further after having complete Methods in year 11, and who end up getting high 40s/50 (even with scores of like 35 in Methods). I'm also under the impression that Further is a lot less stressful; for somebody who has done Methods already, the concepts in it are a piece of cake, and really, you can do minimal study for everything except the exam.
So yeah, in my opinion, if you just want the top score, want to be able to focus on other subjects AND don't really care too mcuh about maths/don't have the time for spesh, go Further. If you genuinely want to learn and would rather have a relatively easy "solid" score instead of a potentially easier "perfect" score, then go spesh.