pretty sure eco scales better. in terms of which one requires more work, to me both are kind of similar, it just depends on what you're more interested in.
modern was fairly easy ngl, as long as you know how to write a good essay, you can write an essay for every syllabus and get a b6, while with eco its a bit less predictable as there's a multiple choice section too (in terms of hsc exams).
in terms of actual content again I think it's each to their own (so unhelpful hahaha), I know plenty who love or hate both subjects. in my experience people who like english tend to like modern since there's a lot of essay writing and it's more about the social aspects of the modern world. eco is about the practical aspects of the world like supply and demand, trade relations- stuff you'll see on the news.
honestly just pick whichever one interests you most, that'll be most beneficial to your atar...but you still have a year to decide so take it easy lol