Like many people here, my advice is the lightweight one. >From my user experience, the "graphical editor" can be really cool to see (not only in tapestry, also in JSF and before in JSP or swing/awt ecc...), but is useful only at the beginning of the project and damaging since then until end of time :D Also, all my projects are imported through maven and i use high level of modularization, so I usually lost all the "tapestry nature" features.
One question: if you detach that from WTP, don't you loose the autocomplete feature for html/css tags inside the TML ? Cheers, Giulio