When reading this post:
I wasn't sure what the last 3 sentences actually are stylistic about.
So the question is as the title states.
What is the expressing difference between We ain't and We are not?
Ain't is a regional dialect word (Cockney, East London, UK- 1706) that started off as a contraction of am not, then was used to replace is not, has not and a few other things. It is relatively common in the UK and in the US. There are other regional variations in the UK- aiy is used in the Black Country and bain't is used in the West of England.
These words are widely used in novels to suggest that a character lacks education: I suspect that's the point of the quote that you referred to.