i do both english extension 1 and history extension for year 12 so i might be able to offer some tips
i agree with carrotsss in that it might be beneficial to at least start both of them and then see
in my experience, although history ext is a lot of work during the major work (writing a full 2500 word fully researched essay + still learning content + all your other subjects) it gets a lot less stressful during the end of the unit when you're just learning content, because most of the content is applicable across both essays and also you can use your major work content if you're smart about it (i.e. i wrote my major work on plutarch, i wrote about plutarch in the trial because one of the sources was about biography)
with history extension you could feasibly get away with - for the whole essay - knowing seven historians bare minimum (purpose, methodology, one or two quotes), whereas english extension you have to (obviously) memorise a bunch of quotes and consider all syllabus dot points, including studying two related texts yourself
totally up to you, but my tldr would be - history extension is more out of class work, especially in the first two terms wherein it becomes pretty much a 2u subject, however the actual exam is quite easy imo and doesn't take too much brainpower - you can adapt most historians to suit most questions
whereas english extension 2 is a bit of a wildcard with exams - you can get essay questions that are quite hard and quite specific to certain syllabus points (unity, justice, restoration, solace, etc), plus literary worlds is mod c on steroids
so i would say if you're better at out of class work/if you want less stress around hsc and trials - history
if you are okay with exams and you want a more consistent schedule - english
again though, there is no wrong answer and this is all just my experience
also one more thing would be that history extension is very different from modern or ancient - it focuses on historiography, or the creation of history, rather than the events themselves - i.e. why would a historian with these views write this history
you may already know that but it's certainly something to consider if you didn't