Hi FutureDR!
In our experience, mainstream English and English Language are two very different subjects! Mainstream English has a lot of breadth, particularly in this current study design, and students have tended to enjoy the range of writing it allows for, so if you're not great at a particular type of writing or analysis, your whole score doesn't hinge on that and you're likely to enjoy another part of the study design a bit more.
English Language is very narrow in terms of its scope - it is all about how the construction of language (note - NOT English, language!) is used to achieve social and cultural purposes. So if you don't enjoy that scope of study, the subject is going to be pretty miserable for you! In terms of the tasks, a large proportion is technically analysing pieces of communication in great depth, and you have short answers, longer answers and entire essays you'll have to write.
We have blog post here about all 3 VCE English subjects.
Our usual recommendation is to choose a subject that you're likely to enjoy (because then you'll be more motivated to work hard at it), and one that suits your learning and writing strengths, rather than looking at whether something will scale well or is more 'score-able'. Whichever way you choose, good luck! And feel free to reach out for 1:1 tutoring if you feel you're in need of extra support. 🙂