The two lenses are really quite comparable, though not exactly interchangeable.
If you're planning on using it for portraiture, the only real difference is that you're going to need a bit more working distance with the 100 to get the same composition, and you'll as a result have a bit more compression of the perspective and get a bit more background blur. Not a huge amount, but you'll be able to tell the difference in side-by-side comparisons.
There're a few favorite variations on the "holy trinity of primes" theme for event (especially including wedding) photography. The classic one is 35 / 50 / 85. Also popular is 24 / 50 / 100. Some who're especially fond of the 135 mm focal length might go for something like 35 / 100 / 135. With the new Shorty McForty and the new 24mm f/2.8 IS combined with the insane low-light performance of the 5DIII, some serious consideration should be given to 24 / 40 / 85. However, if you're not sure, you're far better off doing your event photography with a standard zoom until you <i>are</i> sure.