I've created an ashx page which is going to serve me an XML document full of basic user information. I'm not sure which is the best way to go about creating and populating my custom javascript object. I've seen them created in two ways:

function User() {
   this.Id;
   this.FirstName;
   this.LastName;
   this.Title;
}

and

var User2 = {
   Id: null,
   FirstName: null,
   LastName: null,
   Title: null
}

I could populate each of these by doing something like:

//first object
User.Id = 1

//second object
User2.FirstName = 'John'

Is one method of creating the object better than the other?