- There are no advantages to using heredoc. Except for newlines (you didn't mention this).
- Variables will expand as they would in double quoted strings.
PHP will not allow heredoc to be used in the initialization of class properties, so you could say that's an disadvantage as well (Since php5.3 however, this is only true for variables within the heredoc).
Nowdoc is to single quote, as Heredoc is to double quotes. (Nowdoc will not expand variables).