Hey there lu21 and welcome to the forum!
I'm assuming you're from WA?
Here are some things to consider when choosing a math subject:
- do you need any as a prerequisite for uni courses?
- do you enjoy methods?
I'm not from WA, but the VCE curriculum seems to be similar to WACE. Methods 3&4 is an extension of the concepts studied in Units 1&2. Some people find it easier to jump from 1&2 to 3&4 than Year 10 to 1&2 and some people find it harder. I'm sure many people have achieved a 99 ATAR without doing methods at a similar rate to those who have done methods (I don't have the stats for this though). Anecdotally, someone I know from school only did subjects that scaled down and got a 99+ ATAR (albeit in the VCE curriculum), so yes, it is possible. Doing a subject for the scaling is not always the best idea because if your raw mark ends up lower than you'd like, your scaled score might end up lower than the raw score you could've gotten if you did a lower scaling subject.
Hope this helps you out a bit