 <p>Performance isn't really a consideration, assuming that you're not talking about gigabytes of XML. Yes, it will take longer (XML is more verbose), but it's not going to be something that the user will notice.</p> <p>The real issue, in my opinion, is support for XML within JavaScript. <a href="http://en.wikipedia.org/wiki/ECMAScript_for_XML" rel="nofollow">E4X</a> is nice, but it isn't supported by Microsoft. So you'll need to use a third-party library (such as JQuery) to parse the XML.</p>