I went to both, and my rank went up from bottom in year 11 (from going Matrix ((( I went there from year 7 onwards ) to top 5 in early Year 12 (from going Sigma!!!)
Matrix was alright, however some of the tutors were super questionable when it came to explaining things, which is probably why I completely died in my prelims. I don't really remember much from my hsc year tbh lmaooo but Sigma put more emphasis on how I should be answering/approaching different questions which really helped me/stood out to me!
I liked sigma but if you want to go there you should book early since it's really popular. The teacher is funny and the booklets have lots of questions you go through in class. Also he's a HSC marker so the tips are essential that he gives and he also goes through content more in depth than required so that you can achieve band 6. I got 95 in phys and definitely 100% sure that sigma helped a lot especially with the techniques and the way I wrote answers