My first reaction is that 'excerpt,' or 'passage,' is preferable for text from a novel.

'Extract' might sound more scientific or something, but I can't actually think of a context where you would really use it as the first choice in this sense. Perhaps in something like testing reading levels, or psychological testing? Or perhaps in the context of republishing only part of an academic paper / journal article? Cf. 'abstract' for a short summary of a journal article ... Possibly also in another very formal context such as legal writing, for a part of something like a court decision?

See what others think.