Watt-seconds (a/k/a Joules) is a measure of the amount of electronic energy stored in a system... and it is independant of things like reflectors that channel the energy (converted to light) in a particular direction or the effeciency losses in converting it from electrons to photons.
Guide numbers are a measurement of how much light will strike a subject at a particular distance from the strobe, (and this is the important part) when the subject is in the center of the field of light produced by the strobe.
Consider a bare flashtube with no reflector... light goes everywhere. Now take that bare tube and use a shiny parabolic reflector and you can reflect a lot of light that would have gone different directions into one direction.
For example I can put 2400 Ws packs in the corners of the Dean Dome at UNC, and without reflectors, not have enough light on the court to shoot. But with long-throw parabolic reflectors, I'll easily get 16 times as much light (i.e. 4 stops) on the court.
So in addition to the guide numbers, you have to know the angle of coverage of the flash, particularly if you use wide angle lenses.
So to compare apples to apples, you need the guide numbers for the two flashes, using the same angle of coverage, at the same speed film. It is easy to adjust a GN for film speed differences (multiply or divide the distance by 1.414 for each stop)