The HSC is all about marks, not education (unfortunately). So go for whichever you are better at, to score those marks. You should certainly listen to the different opinions and advices of the BoS members, but dont let them tell you what to do!! Cos your the one whos doin the HSC and u have to make ur own decisions!!!
Remember, performance is all about impression marking - the markers arent gona have the score infront of them. Play something that shows your technical capabilities, preferbly something tonal so they can get involved in the music, and also you have to 'peform' - you have to like fully move around and be fully into the music...or at least pretend to, cos u have to be interesting to watch too, not just listen. Dont play something thats too well known.
Composition is great because you just have to do it, and hand it in, and thats all there is to it. Sure the actual composing bit may be very time consuming and difficult, but once youve done it, you dont have to worry about it, especially if u finish it off well before the submission date. You see, performane is a little risky, cos you peform on the day, and even if you have been practicing 6 hours every day, if u get nervous or sumthing and stuff up your performance, u wont get the marks u wanted... therefore composition can be a safer option... but but but remember that the hsc composition is about clear score notation and development of musical ideas and stylistic understanding. You could write a great sounding piece, but if ur scores rough or something, youll lose marks... so you have to be really careful with composition... Oh and also write something interesting, by using like unusual instrument combinations, or by exploring lots of different instrument tone colour techniques. The HSC markers are sick of marking string quartets with the 1st violin playing the main melody.