If I had the chance, I'd definitely pick the IB over the HSC. Yes it will be harder than HSC but if you're planning to go onto university education doing a technical field, then all the first year subjects will highly overlap with the IB content that you have learned. Not only to mention that you will also have a better foundation of statistics and computing (if you pick those subjects for IB), as well as linear algebra (HSC is 90% calculus FYI).
Also as Drongoski said, you have to pick a Maths stream, a Science stream, a Humanities stream, a language, Theory of Knowledge, etc. It gives you a more well rounded education. In terms of the physics and chemistry content, you will actually be learning proper science content and problem solving. HSC sciences in comparison is more like regurgitating dotpoints of content and how this invention has helped society, etc.