I would suggest considering your proficiency in the English language as the main factor, which will hopefully allow you to make a more informed decision. As you are likely already aware, EAL/D is designed for students whose first language or dialect is not English and who require additional support to assist them to develop proficiency in English. This provides an indication as to the standards and expectations of EAL/D, which tend to be not very high in order to accommodate those students.
On the other hand, English Standard, similar to the rest of the English courses, is designed to be taken by students who are native speakers (that is, in the same way that a student's native language is taught in their country of origin), are fluent in the English language and/or who do not satisfy the eligibility criteria to take EAL/D (such as having studied in an Australian school with English as the main language of instruction for less than five years before commencing the year 11 course). This therefore means that the standards and expectations of English Standard are higher than those of EAL/D, and further increase beyond English Standard.
Based on this, if you have not been performing favourably in English Standard (and assuming that you still satisfy the eligibility criteria for EAL/D), you could consider taking EAL/D. As someone who was eligible for EAL/D (but could not take it due to a clash with another subject) and ended up taking English Standard, I would have appreciated the opportunity to take EAL/D and potentially perform better than I did in English Standard. While you can definitely perform well in English Standard with regular study and extensive preparations, I would personally recommend taking EAL/D and doing your utmost to maximise your results, which could also ultimately be more rewarding due to the better scaling of EAL/D.
On the other hand, if your performance in English Standard has been favourable, then you would also have a good reason to keep it, especially since the EAL/D HSC exam includes a listening component, which some students may be unfamiliar with.
I hope this helps!