What is the Utterances?
Utterances appends a comment block on your website using github repository issues.
Which one is better versus Disqus?
The biggest problem with utterances is that only github account holders can write comments. Other than that, there are no ads and the freedom of comment itself is much higher. (Personally, it's even prettier!)
Who can write?
only github users
You can add it a page, not whole pages using page yaml block
utterances: repo: xxxx/xxxx