I think your HSC Trials are a better, or closer indication of your HSC Mark. Then again, your assessment mark is obviously determined by all HSC Assessments, however the Trial is a better indication of what you might get in the HSC Exam.
It really depends on the school and how hard they make tests, assesments and mark. I think the best thing you can do is to find out what kind of marks people in previous years were getting, and then what HSC mark they got. But then you still have to take in differences in years (ie some years are smarter than others).