Client Side Versus Server Side
Client side processing is in line with the more popular REST standards as well as MVC as opposed to page based approaches and SOAP. The emergence of these trends and focus on AJAX and Html-RIA, client side scripting is on the rise and more popular; however, due to security concerns and client capability, client-side scripting has a particular niche and should not be used for everything.
If a large segment of your target audience will be mobile users, heavy processing should be left up to the server.
Client-side processing is left for the user interface and creating rich internet applications (RIAs). It is used to create animations, effects, user interactions, as well as dynamically loading content through AJAX calls instead of re-loading a whole page.