Hey there
I'm EarthSci34 and I did 4-Units of English for my HSC Year. I enjoyed my extension subjects terrifically, so as a biased response I would encourage you to undertake Extension English. However, marks are not always a fair indicator of your capacity to do well in EX1 as it is a significant jump from Year 10 English. IMO, to do well in EX1 (and EX2 as well) you will need a lot of interest in English as well as a very good work ethic.
For an introduction to Extension English I, it is aimed to challenge and extend your knowledge on how you value and understand various textual forms across a variety of contexts..., may it be historical, as a genre or as a school of thought. In Year 11, you are free to study what the school determines as your "module" (e.g. I did Gothicism, I know someone who did Utopias etc.). Preliminary EX1 gives you a feel for the more difficult and in-depth analysis of language/ textual form as compared to the 2U advanced course (which I think focuses specifically on 1 aspect of a text). In Year 12, you study 1 elective, which is chosen by your teacher (or the school). These electives are Module A (Genre Study) Module B (Ways of Thinking) and Module C (Language and Values).
Despite the similarities present within Adv and EX1, there is indeed a difficult component attached to this subject, especially in regards to understanding the entirety of the text in relation to what you are studying. Compound this with a detailed understanding of your module (with rigorous preparation for essay writing and creatives), it ultimately results with a challenging course that consumes much of your time. It is developing a personal understanding of your elective that defines the higher-end marks and allows your responses to be authentic and well-rounded.
It is a demanding course... but ultimately quite rewarding (please disregard the scaling bit of EX1, I'm talking about beyond marks). You garner a new found appreciation of textual forms and features as well as increased insight into the significant value of texts. Sure, the workload gets a bit intense. That said, if you really do enjoy English then it will be something to look forward to! Besides, you can always drop if you do not feel the subject as useful or too difficult. Also, EX2 is a possibility-- keeping your options open
Best of luck with your decision! I had the same dilemma in year 10... *oh memories*