I throw in my vote for justified text. Since Gutenberg, the vast majority--overwhelming majority--of books have been justified, and I suspect the majority of readers expect to see just that. It's possible to format an e-book to minimize the ugly-gap problem, which anyhow is vanishing as e-readers become more sophisticated. I see few professionally formatted books on my Kindle or Fire that suffer from this problem.
And a book is not a web page. Not yet, anyhow.
@OP: You can see my style sheet at
http://notjohnkdp.blogspot.com/
If the style sheet exists in the same folder as the underlying html file, Sigil will incorporate it into the epub.
Good luck!