The benefits of doing Advanced English and/OR English Extension 1 OR even Extension 2 are very clear ...the big dilema is whether to choose between Standard and Advanced, not whether to choose ENGEX or not.
Obviously, if you get A's or B's in Year 10 English, you are a student that's very capable and you have the ability to achieve a BAND 5 or BAND 6 in HSC English.
My Year 9 English Teacher, who also happens to be the school's English Coordinator, Mrs Sewell, said,
"Advanced is still quite a difficult course, but Extension is extremely challenging [for students who don't do well in Advanced English]."
So, yeah. If you want to do Advanced English and you are a B student for English, then you have to work harder than an A student would. SIMPLE.
I reckon: - If you get an A in Year 10 English Semester 1, do Advanced, and if your teacher says that you are "capable", do English Extension.
There's no point in dumbing down just to be lazy so that you can do well in 10 units and not count English to your UAI, ENGLISH SHOULD COUNT.
It is the most useful subject (if you end up working in an English speaking country), as you will have to speak and understand English in every single job that you ever do.