I haven't heard more than one person say they had trouble installing kde, but gnome installed ok. In any event, if it was me, I would probably get this one:http://torrents.sabayon.org/file?info_h ... 00%C5KU%93
which is the gnome 64-bit iso (version 13.09). and upgrade it, and then install kde:
equo install kde-meta
I suppose installing gnome from kde is similarly easy, but, I've never done it, and don't know how.
I doubt that USE flags are going to enter in to situation at all, unless you decide to build your own kernel or start using portage.