Hello!
It sounds like you would love doing both! You can always drop one after Year 11 (especially as you said you were completing methods 3/4 in year 12) and it would give you a range of options as well

. English Language is very broad, especially in 1/2, where we have learnt about grammar and the function of language, child language acquisition, the history of English and English on a global scale. Mainstream English is very similar to Year 10 English (at least at my school). The assessments are text response, comparative, analysing argument, point of view oral and a creative piece.
If you found another subject that you want to take, therefore only taking one English I would recommend English Language as you can always hop into Mainstream English at year 12, but not vice versa.
I hope this helps !
EDIT: I just realised you said that they were due last night but nevertheless I hope this can give you and possibly some other people some insight 