I went with the 100L because it's a two trick pony and has IS. And impressive IQ. That made it incredible bang for the buck.
That said, I'm still attracted to the 135 and at some point will try it. It 'has that look' and also grabs more light and has more reach...good things for shooting stage, which I do. The big downer for me is no IS.
I don't want to start another debate of IS vs no IS, but just say that if your hands are as wobbly as mine, it helps in most every situation. I (unfortunately) easily motion blur a 1/200th shot with a 100mm lens and no IS enough to be noticeable, no matter how hard I try to 'bead the target'. Which is so strange since I'm so proficient with firearms...I suppose too bad for me that viewfinders don't work when holding a camera with fully extended arms...lol maybe I need to put a gun sight on top of my 5D2 so I can use all of these wonderful non IS lenses? But in all seriousness, IS and closeup would be the reasons to pick the 100L over the 135L IMO, given the length difference is not enough to matter to you. Otherwise 135L.