Tough call.. And what I'm about to write is very subjective. I look at lenses in a very different way then most.
The 120S is better for beauty and portraits. I say this for two reasons. One the perspective flatters the face a bit more. And two, the sharpness and roll off is a bit better suited for faces. Less character, so less distracting. A cleaner, fresher look if you will. Besides there's something about the contrast that just gives off a sense of realism (though it fades the further away from your subject you get).
The 100S is better suited for half body and beyond. Especially candids and street scenes. And it makes a great lens for shooting details and close up quirks etc. It also makes a better lens if you're shooting multiple heads in one frame. The rendering also tends to be more in line with the older sought after Leica glow look. And it's just beautiful. How lenses should render
Reminds me of a mix between the 75APO and the 75Lux. Basically the good points from each lens sexed (my wife laughed at me when I asked if sexed was a word, as it didn't look right.. She said the word I'm looking for is mated) to create the 100S
It's not to say both lenses can't do the others job. It's just to say that I wouldn't want to be without either lens.
As for the actual real world use. The 100S will be faster, even when the lens has less distance to travel. As in, even if you're taking the same subject and neither you or the subject are moving much. Obviously from close range to far away the 100S is VERY fast in comparison.
There are two exceptions to the rule.
1) On the S007 the 100S can be a bit problematic. Sometimes it just won't lock focus, and I've tried with multiple 100S lenses and multiple S007 bodies (one of the reason I don't have an S007 anymore). And even though the recent firmware fixed the issue a bit. It's still problematic. In this instance, the 120S is a faster lens.
2) In certain lighting conditions the 100S has to many internal reflections (poor flare control) and it won't hit focus. Whereas the 120S seems to have less of an issue with this, both visually (in images) and in operation.. The 120S will in that instance focus faster as it doesn't struggle like the 100S does.
On a whole though, the 100S could be used for slow-ish moving subjects, whereas the 120S would be reserved for static subjects.
Hope this helps.
P.S. One more thing to add.. The 70S shouldn't be overlooked. It does great in backlit situations and has beautiful rendering. If you consider it, you'll probably want to space it out a bit and the 120S is a good friend for it
Also in that vein of thought, you might want to pickup the 35S instead of the zoom