135 is bigger, heavier, and a more expensive - almost twice the price.
135 will have slightly better colors/contrast and slightly more attractive background blur (bokeh).
100mm vs 135mm is a noticeable difference in focal length and is something you should consider.
Both are very good lenses - sharp, and fast to focus. The 135L is the better lens, one of Canon's most highly regarded. The 100mm is the better value. The difference is not as dramatic as the price might suggest.
You might also put the 85mm 1.8 on your list, it's basically the same as the 100mm f/2, just a little shorter with a little faster aperture.