Lighter roasts are better for espresso and that's what I always use. Roasting dark causes the oils (and flavor) of the coffee bean to come to the surface and much of it is lost to evaporation during the roasting process. For drip coffee, this is OK because when the oils are released from the bean, more can be extracted during the drip extraction process. The espresso process has the ability to extract the oils from within the bean, so dark roasting is unnecessary and counterproductive.