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.
the debugging feature is actually also important - however I would like to treat all these libraries as bugless, full featured, and reading my programming mind things ( :-) ) I find that more often than not I need to peak here and there under the covers to understand what is going wrong with my stuff.
bottom line, I think both libraries gxt and gwt-ext are great and at this stage of RIA technology development should be considered without regard to what is duplicated where and how one widget/API is different or similar than the other. As a consumer of these technologies, the more of the alternatives the happier I will be.