« on: April 25, 2013, 04:40:08 PM »
I also own both of them, I had a 100 non-L at first but wanted the IS, so got the 100L instead. I bothered quite a time if I'd need the 135L. Though they are not far away from each other regarding their focal lengths, they are both useful in their own ways. I used the 100L for macros a lot in the beginning (obviously) but also as a portrait lens. Also I have it with me, next to a wider lens, when I am only hiking around or so because it got an IS, and it's weather sealed while the 135L is not. Keep that in mind for snow or rain, I had a shoot with a girl when it snowed heavily and I could have cancelled that without the 100L. I use the 135L almost only for portrait work and then from a tripod.
The 135L is a bit better regarding sharpness and image quality but they are still both very, very sharp, probably the sharpest lenses in that price segment. If you can afford it, get both, unless you only plan indoor use or in very good weather conditions / at daylight, then I would consider the 135L.