I think there is some difference between the xajax & json extensions. The xajax extension packages the xajax project ( http://www.xajaxproject.org/ ) and provides access to an ajax library. It doesn't actually do anything itself. It's a great starting point for adding ajax functionality to a site.
From what I've read, Xaviers JSON extension will provide access to eZpublish objects (nodes, objects etc) via ajax (presented in JSON format). This sounds a little more specific to me.
So xajax would be a great base for building any ajax functionality, JSON sounds like it will be excellent it you need to deal with eZ Objects via ajax. (While I haven't yet seen the JSON code it could be built using the xajax library)
@Xavier - I think you need 2-3 more *s in the JSON project description ;)