Honestly, I don't see it. It's been a while since I used emacs for development, but the entire reason I moved from emacs to eclipse was because I had at least a 10x improvement in productivity working with large [java] code bases.
Has jdee come a long way since? I just checked and it doesn't look like it.
As far as java on emacs, I can't exactly comment. Java is my one main exception to emacs. Its not that I dislike it on emacs, I just have never tried it there. I learned on Eclipse it worked for what I needed (although Eclipse is horribly slow...).
Has jdee come a long way since? I just checked and it doesn't look like it.