To be precise, the vast majority of SE is from Gutenberg, but we also source from Faded Page, Gutenberg Australia, Wikisource and occasionally do our own transcriptions.
Personally I find the formatting used by the Gutenberg one to be a lot nicer/easier to read, despite (or perhaps because of) being simpler, more plain.
At least for the first few pages of content that I looked at on both versions.
Technically, I can also just directly pull the epub from Project Gutenberg, but sometimes the formatting leaves a lot to be desired.
Once you get an e-reader that runs a semi-capable OS (ex - stock android, even an older version), it's hard to go back to something like a kindle.