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.
I faced similar dilemma few months back. As Lennart (above) says it makes sense to go for pyjamas or similar library if your app is more desktopish.
Further one of the biggest advantage of pyjamas provide is logically well separated backend and frontend code. IMO that is very important. 
If your app is not like a desktop app (like ours was), then multipage offers more advantages, such as single change wont breaks entire app, easier to maintain etc. You might want to consider can have your app server serve json and other web server serves static content and js. Js would request json app server for data. That way we managed to keep out frontend and backend separate. Further we chose mootools as js lib over pyjamas. Ofcourse it is upto your taste and need of application. We did use python template server side templates but at compile time not at runtime like usual approach. This needed to change our thinking a little but the offered many advantages. 
I end up telling you my story but I thought it's relevant and hope that helps.
rich web client vs thin web client