Both approaches have partly a different scope: xajax is easy to expose kernel/your php classes and functions to Javascript, JSON is more a data transport mechanism
With eZ Publish 4.x, expect more JSON use ... so you can say we adopt it indeed.
I'll test this out locally to see if our simple xajax scripts are as simple with json, with xavier's extension. I've really enjoyed using xajax as no-fuss method of ajax-ing with eZ.
