Which of these styles is nicer?

es6-styles.js

/**
 * Returns server environment information and copyright. This is intended
 * only to be displayed inside of a terminal.
 * 
 * @returns {String} Copyright and server environment information
 */
function getServerInfo() {
  const engineName = process.title == 'node' ? 'Node.js'
    : process.title == 'iojs' ? 'io.js' : '(unknown engine)';
  const engineVersion = process.title +  ' v' + process.versions.node;
  const enginePlatform = process.platform + '-' + process.arch;
  
  return (
    copyrightInfo.string('source', true) + os.EOL
  + version + os.EOL
  + 'Running on ' + engineName + ' ('
  + engineVersion  + ', ' + enginePlatform + ')' + os.EOL
  );
}


/**
 * Returns server environment information and copyright. This is intended
 * only to be displayed inside of a terminal.
 * 
 * @returns {String} Copyright and server environment information
 */
function getServerInfo() {
  let output = '';

  const engineName = process.title == 'node' ? 'Node.js'
    : process.title == 'iojs' ? 'io.js' : '(unknown engine)';
  const engineVersion = process.title +  ' v' + process.versions.node;
  const enginePlatform = process.platform + '-' + process.arch;
  
  output += copyrightInfo.string('source', true) + os.EOL;
  output += version + os.EOL;
  output += 'Running on ' + engineName + ' ('
    + engineVersion  + ', ' + enginePlatform + ')' + os.EOL;
  
  return output;
}