Simply put, ExtGwt is GWT that looks like Ext. While GwtExt is Ext wrapped in Gwt. Sencha chose to build their world on top of the true GWT instead of wrapping their legacy Ext into GWT. A great choice!
I have nothing, personally, against Ext GWT but just looking at the examples (http://www.gwt-ext.com/demo/) you will IMMEDIATELY see the difference: Gwt-Ext is much more complete than Ext GWT.
Furthermore using Gwt-Ext you will get:
– GWT wrapper for ExtJS user extenions (http://gwt-ext.com/demo-ux/)
– Supports for charts (using Yahoo Chart API, http://gwt-ext.com/demo-charts/)
– Supports for muktiple maps API (Google Map, Yahoo Map, etc. – http://www.gwt-ext.com/demo-maps/)
There is a difference between the number of widgets int Ext GWT and GWT-EXT. This gap is decreasing as many more widgets are on the way. Also, keep in mind there are many Ext GWT features that are not in Ext JS or GWT-EXT, such as its viewer and mvc code. Ext GWT uses GWT 1.5 and takes advantage of Java 1.5 using generics, enumerations, varargs, etc.