I have one design decision to make. 
In my web(ajax) application we need to decide where should we put user interface logic ? 
Should It be completely loaded via javascript ( pure single page ) . and Only data comes and go.  
or 
Should server send some format (XML) which translated via javascript to dynamically create rich user interface. ( semi-ajax ). so some data and ui comes and go. 
Which option is better ? ( speed, ease of development, platform independence ) 
Thanks.
The biggest influence is whether you are concerned about initial page load time. If you don't mind having all the UI there at page load, your app can be more responsive by just shuttling data instead of UI. If you want faster load and don't mind larger AJAX requests, sending some UI markup isn't bad. If you have the server power to pre-render UI with data and send the fully-ready marked-up data to the user, their browser will perform more quickly, and initial page-load should be fast.
Which course you choose should depend on the task at hand. Not all requests need be handled the same way.
rich web client vs thin web client