For y12 I find that holiday classes have really helped so far (I went for term 1) because I was able to understand concepts easier at school for Chem - I just had to constantly practise/spam/revise questions for a majority of the school term and go to workshops at Matrix if I needed some help. You also have to consider the fact that your school might teach faster than Matrix (but for my school, Matrix has a slightly faster pace), so that might be a problem during the term.
But I definitely found the holiday course way more exhausting than weekly classes since I did both Maths and Chem in the holidays last term...and that's like 6 hours a day for 2 weeks straight, plus additional hours of daily homework and stuff. Pretty much school...but way more learning involved.
Well, personally, I've performed better in Y12 after doing the holiday course as opposed to how I did in year 11, when I went to matrix during the term. I feel like they'd be the same thing - it just depends on how well you retain information, since they both offer the same information and assistance I guess??
Anyway I'd say go to at least one in the holidays and the other during the term maybe and just book workshops if needed when school starts again