The HSC and Prelim courses are designed to be similar in terms of themes and such.
What specific areas did you do in your Prelim course? What would you be doing if you did the HSC course? Find out, and perhaps make a decision on that.
So things to consider:
* What areas you're studying and if you like them
* what teacher you'd get,
* whether friends are doing it,
* what kind of marks you got,
* what your teacher says - have a chat with them.
Some of those things seem trivial, but if you have an awesome teacher, it's hard to have a bad time!