The staining as outlined by Jim is the problem. Baseboard heaters are usually made in two watt densities, that is the amount of electricity used per foot. Usual is 187 or 250 watts per foot. The lower watt densities, spread the heating load over longer sections, thus run at lower temperatures and the air moves across them a lower velocities, carrying less debris (which is what sticks to the walls) All of them, including hydronic heaters, causes some wall staining but the lower watt-density heaters will cause less. Spring cleaning each year will minimize the build-up.