The basic summary is that aD is moving to Java since Java is more buzz-word compliant. It is (perceived to be) easier to sell to big companies than Tcl. However, aD still will have a lot of client projects in Tcl, and at this point ACS 4 is more advanced in Tcl than Java. The tcl version of ACS will likely continue living on in OpenACS long after the final aD version in Tcl.
Since this is an open source project, the people interested in helping with the Tcl port will help with it, and those interested in helping with the Java port will help with it. I'm currently much more interested in Tcl than java, but then since my focus is on getting things done and done quickly (I'm not in marketing, so I don't need to worry about being buzzword-compliant). Different people will surely have different opinions.