In practice, there seems to be no difference.
From what I understand, most people and IDE implementations prefer to instantiate in parentheses to follow the standard code used in PHP’s own documentation and by the wider community, which follows that of other object-oriented languages.
I couldn’t find a reference in the PHP documentation, just one in Wikipedia what it says:
Function calls must have a parenthesis, with the exception of the class constructor function when it has no arguments and is called with the operator new
PHP, where parentheses are optional (free translation).
One might argue that by saving the two characters (2 bytes if using ASCII encoding, 4 if it is a UTF-8, ...) your PHP file would be loaded and interpreted faster. However, any gain will probably be insignificant and not measurable, since PHP files are always full of "leftover" characters, especially when mixed with HTML.