rich web client vs thin web client
Which option is better ? ( speed, ease of development, platform independence )

Platform independence, if you mean cross-browser compatibility, is a HUGE reason to use pyjamas because the python code includes a sane override infrastructure which handles everything for you. No more JS compatibility classes.
Anyway Pyjamas is all about loading the client app and then using json-rpc for the data only. That's because it's faster (once the app loaded up), easier to separate server and client, easier to maintain since all the UI code is in widgets in one place.
I've seen stuff like DokuWiki which use a php script to serve up javascript and my first thought was "WHY?" but it works pretty well I guess. It probably makes sense if you mostly have static pages with the occasional bit of JS for decoration.
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.