Assuming that both containers are identical in size, shape, and thickness, the glass one will keep food warm longer. Glass is a considerably poorer conductor of heat than is metal, and therefore the rate of heat loss through the sides of the container is slower. The same principle is true for glass's close relative, ceramic.
Many other variables also affect heat retention, including the food's fat and moisture content. Fatty foods stay warm longer than lean ones. Ditto for solid foods versus liquid ones like stews. Of course, the most influential variable of all is whether the food is covered.
** Asian Recipes