i'm totally unqualified to give advice on this, but i did bio last year and picked up hhd this year and they work really well together tbh. there's definitely things to memorise (so it's not a bludge in terms of content – nothing is complicated, but you need to know it. although i've heard they're going to be heaps less strict with this is the 2018 study design which is pretty good!), but the majority of the marks are in the application questions and those are mostly logical, so it's kind of like bio in that once you learn how to answer the question, it's not super hard to actually do. there are graphs in health too, which a lot of people seem to find kind of hard, i guess having not done them much before? haha. plus genetics wanders into unit 3 a few times, which is nice. but again, depends on what you're looking for in your extra subject (: