As a qualified mathematics teacher and someone who went through the after school tutoring program during High School, I would suggest that you do
Mathematics Extension 2 (formerly 4 Unit) if you have a passion for the subject and can show a strong degree of creative thinking. The mechanics and
conics sections in my opinion require a lot of unconventional methods to solve the questions, as its really more algebra rather than a direct application
of physics and conical equations. You must also be highly committed, willing to spend a lot of time studying mathematics and doing after hours
tutoring.
I did Mathematics Extension 1 at school and found it was reasonably challenging, but more straightforward in the problem solving approach compared to Mathematics Extension 2. If I had to do school again, I would have picked Mathematics Extension 2, despite my reservations about the conics and mechanics sections. So far I have found most of the topics like complex numbers, graphs, polynomials to be more methodical in the problem solving