It pains me to say this (genuine, stabbing, excruciating pain) but from what I can see bio is probably the better choice for you.
I have no experience with bio, but I can verify what emily said in that Modern takes up a lot of time for most people. I like to think that it is because modern history is super interesting and a lot of people are actually willing to put in that much time and effort (I know that was the case for me - i would have rather spent six consecutive hours doing modern past papers than 5 minutes of economics multiple choice during my HSC year), and it can be very subjective (however I do think that there are ways to appeal to most markers in regards to structure for the source analysis and essays - if you do decide to stick with modern defs head over to the modern threads for some tips
).
As far as I am aware NSW uni's do not have prerequisites, however I would assume that bio would be assumed knowledge if you wanted to get into medicine, whereas knowing that the schlieffen plan failed due to Moltke's weakening of the right flank won't be
Bio would also mesh quite well with some of your other subjects (again I am assuming here - I lump all maths and science into the "things I can't do" category of my brain), like Chem, PDHPE and 2U maths, so study wise it might be a bit easier/less jumbled. On that note though, Modern might be a nice break from those kind of subjects, but I feel like 4 unit english probably does that well enough anyway (judging by your subjects you're an extremely well rounded individual academically well done!)
Anyways TL;DR, I agree with emily and Shadowxo, and think Bio is probably the better option for you (refer to this youtube vid for a visual representation of my emotions in regards to this
https://www.youtube.com/watch?v=-pVT_mvvZLo )