Wow you did the exact same prelim subjects as me, so I remember my prelim experience with all of them.
I did both Bio and Modern and they were actually my best and second best subject respectively(although tentatively it was only 1 mark difference, and iirc Modern scales slightly better which makes it fairly equal). They are both quite different in their demands and their skills, are you performing equally in all your subjects? (Cohort sizes, rank and school rank?)
I'd say definitely try your best in all of them and make the decision after you receive your final prelim ranks after yearlies. Don't foresee the decision and neglect the subject because you think you will drop it(I did this with Economics to an extent), because if you change your mind (and you do tend to in your way through yr 11), you may regret it. Putting a good effort into all subs in prelim will give you a good gauge of where your capabilities lie and which subjects you will reap greatest reward in (effort vs reward).
On the whole, I'd say most people find bio easier in the content as you can to a degree rote learn parts of the syllabus. It's also more 'common sense', the syllabus is easy to follow and you can literally tick off dot points. However to reach the upper echelons of bio marks, you have to pay strict attention to the verbs and structure of your answer. (Especially Discuss questions). Also you have to clear and structured. Ohhh and Yr 12 bio content is far, far more interesting imho, if you like human biology over plants and that.
More people find the content in Modern overwhelming (I personally did not so, but I think many people did not realise you didn't need to know every XYZ fact). What the real crux of Modern is, is structuring coherent historical arguments with good essay skills. Source work is also a key element. If you do not grasp the holistic crux of whatever topic you are doing, you will struggle particularly in the National and Conflict study. I'd say with Modern, definitely check with what topics you are doing in Yr 12 because even good history students have topics they love and ones they are quite indifferent too. We did more interesting things in our Yr12 course, but only because I loved Russia.
I'd say to succeed in both, you need hard work but interest can be a big factor in which you perform better in. Interest is a good decider if you have equally apt skills for both of them (like I did for Bio and Modern).
Also out of interest, are you planning on picking up any units in Yr 12?
