DW its early days still, you've plenty to time to make your decision. I remember back to the early days in year 10 when i couldn't choose between which subject (either eco or modern coz i did quite well in commerce and history in junior years) to pick for senior school and then i thought which subject i enjoyed more, got better marks in, the workload involved in yrs 11 and 12 and if there was anything in either subject that would help me in the future to whatever uni course i do. In the end, i chose economics and don't regret it at all, but if i were to do 14 units i would definitely pick modern hisytory as my 13th and 14th units. But hey, everyones different, and whatever decision you make, i hope it comes from your heart coz everyones different.
Whichever one you're worse at. Stop worrying about scaling, and if you hate them both the same, then just pick the one you're better at. Eco scales slightly better, but that means nothing if you don't get a good mark...Also, there's not a huge difference between the scaling of modern and eco anyway.
for me modern involves mostly reading and answering questions on sources etc. the assessments aren't particularly hard but you do have to do the reading beforehand to get a good mark. with economics i've found that you have to do a lot of questions and essays primarily to achieve a good mark. i find economics more challenging but probably more rewarding than modern. plus i'm planning on doing economics at uni .
to do well in modern basically memorise a bunch of dates, some historians' quotes and be very good at essay writing. Eco = more skills...gotta be good at essays but also more structural and scientific thinking.