personal preference, graphite is a bit more tolerant of "lack of proper preparation" and will seal small imperfections, however, I use both and have no issues with either. MLS are perfectly fine IF the surfaces are clean and flat. Graphite are just a "lazy" mechanics gasket if they can't be bothered to clean things perfectly.
I won't use a graphite gasket on a high compression race engine, too unreliable. Domestic engines, fine.