The SGS is a Self Guided Spectrograph, meaning when used with an ST-7 or ST-8 an image of what's falling on the slit is shown on the Tracking CCD while the spectra is projected onto the Imaging CCD. In this way you can guide the star you intend to measure. Also the SGS has two gratings, a High Resolution Grating (1 angstrom per pixel) and a Low Resolution Grating (4 angstroms per pixel). With it's Self Guiding and High Resolution the SGS is good at stellar spectroscopy.
The DSS-7 is an classical spectrograph (not self guided) with a resolution of 5.4 angstroms per pixel. Due to it's lower resolution (and thus higher sensitivity) it's best for diffuse objects.