i guess from my experience it depends on what other subjects you are thinking of doing; are they time-consuming/require memorisation (eg. human biology), or are they science-y like physics and chem? if you have the time to focus on methods and spec then go for it. you could do spec with methods in y11 semester one and see how you go - if it doesn't work out, you can always drop down to just methods or just apps.
i did methods in y11 and it was very hectic for me, yet all my life i've gotten 90+ for maths (up until end of y10). the problem with me was that i perceived maths as a by-subject and not one of my main issues, which ended me up on a <60 avg so i dropped it for apps in y12. this was bc i had other content-heavy subjects i had to focus on, so apps ended up being something i studied for just before an assessment, instead of a daily everyday thing.
it depends on you and what you enjoy doing, honestly. try to take all factors into account; ask your teachers who may be able to predict your outcome; and chat with your family members, or friends, who've been through ATAR as well.
all the best!
