More important than whether to send html, json or xml is WHEN to send that content. People underestimate just how painful latency is to the user because they're sitting right next to their web server. Digg takes 15 times longer to load from bangalore than from california (see http://www.slideshare.net/kkjjkevin03/geographic-distributio... ). The thing that client-side rendering allows is caching your application UI in the browser and on the CDN. The server only gets contacted when it's absolutely necessary.
If you go mobile, it gets worse, because sometimes you'll lose the network for a few minutes, or even a few hours. Building a web app that keeps running in that scenario is possible, but only by embracing a client-side philosophy. In my personal opinion, mobile is going to be the dominant way that people connect, and the network is not going to be robust enough to render mobile web apps on the server.
If you go mobile, it gets worse, because sometimes you'll lose the network for a few minutes, or even a few hours. Building a web app that keeps running in that scenario is possible, but only by embracing a client-side philosophy. In my personal opinion, mobile is going to be the dominant way that people connect, and the network is not going to be robust enough to render mobile web apps on the server.