I have the good fortune owning both lens, and I will confess that there is some overlap. I would definitely agree that the 100L is the more flexible tool, particularly if you are shooting a crop body only. In fact, if you only plan to shoot crop bodies, I think the 100L wins hands down.
If you are shooting FF, however, the 135L produces the finest images this side of the $6000 200 f/2L IS. I bought it because I realized that my favorite shots from others were almost always with it. If you are shooting environmental portraits, it is a sublime instrument. I have shot portrait sessions in terrible light and ended up with gorgeous results. It has amazing delineation of your subject from the setting, and the transition to ooF area is incredibly smooth. I use it a LOT in event work like weddings/business events, and it never fails to transform something mundane into something far more special. I also use the 100L at event work, and it is also great. But the 135L is something just a little more special.
Jesse said the 100L is a better value portrait lens. I would agree only if you are talking crop format only. If you are talking full frame, the 135L is arguably the best portrait lens in existence.