`—` or `—` is there any difference in HTML output?

SGML parsers (or XML parsers in the case of XHTML) can handle — without having to process the DTD (which doesn't matter to browsers as they just slurp tag soup), while — is easier for humans to read and write in the source code.

Personally, I would stick to a literal em-dash and ensure that my character encoding settings were consistent.

They are exactly the same character. See: http://en.wikipedia.org/wiki/Dash

Barring browser bugs they will display the same in all cases, so the only difference would be concerning code readability, which would point to —.

Or, if you are using UTF-8 as a charset in your HTML document, you could enter the character directly. That would also display exactly the same.

— :: — :: \u2014

When representing the m-dash in a JavaScript text string for output to HTML, note that it will be represented by its unicode value. There are cases when ampersand characters ('&') will not be resolved—notably certain contexts within JSX. In this case, neither — nor — will work. Instead you need to use the Unicode escape sequence: \u2014.

For example, when implementing a render() method to output text from a JavaScript variable:

render() {
   let text='JSX transcoders will preserve the & character—to ' 
            + 'protect from possible script hacking and cross-site hacks.'
   return (
     <div>{text}</div>
   )
}

This will output:

<div>JSX transcoders will preserve the & character&mdash;to protect from possible script hacking and cross-site hacks.</div>

Instead of the &– prefixed representation, you should use \u2014:

let text='JSX transcoders will preserve the & character\u2014to …'