Actually neither kind of watt-second rating tells you very much in terms of how much light is produced because it is only a measure of the energy applied to the flash tube, and does not take into account the efficiency of the tube, or the effect of the reflector that directs the light output onto a subject. For instance if you plan to use a soft-box or umbrella they are both relatively inefficient in terms of how much watt second energy results in the amount of light on the subject. So you really need to know what aperture you want to shoot at and the effective film speed, and then choose the kind of reflector used and distance from light to subject to discover how much flash power is needed.