I'm doing Specialist and an extension study (biology) this year, after having completed 3/4 bio last year. I find spec more challenging than methods, but also more interesting. The topics we do are very different from ones covered in previous years, while methods has the usual graph drawing, some trig, calculus and probability. I am getting about the same marks for spec and methods, but I do tend to spend more time doing spec homework. However, the competition in specialist is alot tougher. It is mainly made up of 'talented mathematicians', whereas the methods cohort is alot more varied.
My extension study is fairly easy. I don't do much work for it and still achieved a decent mid-semester mark. Between the two, I would say that specialist takes up more time. Think about the type of work you prefer, maths is pretty systematic (just practicing lots of questions) compared to your other subject choice.
I would follow your teachers' advice and enrol in both subjects, then keep the one that you enjoy the most. That was my plan but I decided to keep both!
Good Luck!