This might be dead thread, but I would just like to voice my opinion.
First I would like to say that I am a new user here, but I have enjoyed reading the post on this forum for a while.
In my limited experience, the only reason to consider either or on the 100L and the 135 is cost. I have both, and not long ago I was in that very situation where I had to choose. Now I am in the lucky situation that I have both. The 135 is probably my favorite lens of all I have, and it makes portrait shooting both fun and easy. The 100L is good for portraits too (although given that I find the distance of the 135 great for theese shots, since I feel you get more natural shots without getting in the subjects face), but my main use is for macro. I long considered the 180L for macro, but I went for the 100L after reading reviews on photozone.de and the-digital-picture.com
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Gear: 5DMKII, 50mm 1,4, 70-200mm F2.8L IS II, 16-35mkII, 17mm TS, 24-105mm, 135mm, 100mm F2.8L IS, Tokina 17mm.