The subjects could not be more different and it completely depends on what you prefer and how you are going in each subject. My advice is to keep both for a while, until you have a more clear idea of which you can succeed at. I think that you are more likely to go well in something that you enjoy than something that you hate.
As an Ext 2 English student, I will say that the subject can be a fun 'escape' from other sorts of study over the HSC year. It is totally different to other courses, a great creative outlet, and best of all there is NO FINAL EXAM. So for me, the decision would be easy. But I suppose you can be more assured of good marks in a maths based course, because there is a right and wrong answer, so if you master the technique, you can get the marks. With English, you have to figure out what it is the examiners are looking for.
Hope that helps!