We report on the plasma-assisted molecular beam epitaxy of high-quality Ga-polar and N-polar GaN layers on Si-face and C-face SiC substrates respectively. X-ray diffraction peaks are intense and narrow, symmetric in the theta-2theta configuration, indicating that the relaxation of the GaN grown directly on SiC occurs faster than in the presence of an AlN buffer layer. Photoluminescence experiments performed on GaN layers with N-polarity reveal more intense and narrower emission lines than in their counterparts with Ga polarity.