As far as I’m aware, the use of Maths in Focus is appropriate when acquainting yourself with the concepts, whether it is through consulting the explanations of particular concepts, or when attempting questions relevant to those concepts, allowing you to become familiar with Mathematics Advanced content. As someone with rather modest mathematical capabilities, solely using Maths in Focus allowed me to achieve a band 5 as my HSC mark in Mathematics.
On the other hand, Cambridge appears to be appropriate when seeking more challenging material that extends beyond the difficulty offered by Maths in Focus. If you aim to maximise your performance in Mathematics Advanced, using Cambridge would be a good idea. Additionally, if possible, you may wish to concurrently use both textbooks instead of just one of them:
- Using Maths in Focus as you first encounter a particular concept can assist you in familiarising yourself with that concept.
- Subsequently, using Cambridge will enable you to master that concept, thanks to the exposure to more challenging material, which constitutes good practice.
I hope this helps!