Node.js' http.Server and http.createServer, what's the difference?

Based on the source code of nodejs (extract below), createServer is just a helper method to instantiate a Server.

Extract from line 1674 of http.js.

exports.Server = Server;


exports.createServer = function(requestListener) {
  return new Server(requestListener);
};

So therefore the only true difference in the two code snippets you've mentioned in your original question, is that you're not using the new keyword.

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For clarity the Server constructor is as follows (at time of post - 2012-12-13):

function Server(requestListener) {
  if (!(this instanceof Server)) return new Server(requestListener);
  net.Server.call(this, { allowHalfOpen: true });

  if (requestListener) {
    this.addListener('request', requestListener);
  }

  // Similar option to this. Too lazy to write my own docs.
  // http://www.squid-cache.org/Doc/config/half_closed_clients/
  // http://wiki.squid-cache.org/SquidFaq/InnerWorkings#What_is_a_half-closed_filedescriptor.3F
  this.httpAllowHalfOpen = false;

  this.addListener('connection', connectionListener);

  this.addListener('clientError', function(err, conn) {
    conn.destroy(err);
  });
}
util.inherits(Server, net.Server);

According to the docs, it seems to be

http.createServer = function (requestListener) {
     var ser = new http.Server();
     ser.addListener(requestListener);
     return ser;
};