(I only do chemistry, if it matters) I'm sure ZhangsHSC is a really good tutoring centre (considering the prices and its success rate at producing high marks), it's just a matter of how high you're trying to aim and how much money you're willing to shell out. I go to Proed and I think it's a great place, I manage to do just as well as my peers in who are in Zhangs. If I put in the effort, I'm sure I can outdo them.
I'm sure Zhangs has some amazing way of churning out high marks (I've never been there so I don't know what the environment is like, although I've heard its really good), but unless you absolutely want and NEED 100s and you can afford it, I think you might better off going Proed and being a bit more disciplined. Zhangs is a lot of money for a tutor, and it really adds up. If you choose not to go with Zhangs, that's fine, because I promise you there are thousands of students who B6'ed chemistry/physics without going Zhangs.
In my experience: Proed is nice, pretty chill, and the tutors are great. It's 2hr sessions in yr11 and 3hr sessions in yr12. I'm guessing Zhangs is going to be more rigorous, so if that's more beneficial to you then you should absolutely go for it! If you think you can reach your goals without spending the extra ~$40/hr (iirc) that Zhangs requires compared to other tutoring centres, then all the better!
Again, it's just a matter of where your priorities lie, and what fits your goals & lifestyle better!