Maybe your subjects are different from mine, but my IS zoom is way better from 1/40 to 1/125. Sure, I have had sharp pictures from my 135L at 1/60, but those are rare. I do not have confident handholding the 135L below 1/160, as I can clear see how shaky I am in the viewfinder.All I can tell you is that my 135/2L with a Canon 1.4TC is sharper
and easier to hand hold with both my 5D and 20D bodies than my
70-200/2.8 IS was before I got rid of it.
For slow subjects, the IS zoom is actually a better lowlight lens if you think about. The 135L only has one stop advantage, and it is very hard to shoot below 1/125 without support. The IS zoom can be easily shot at 1/30 by a mere mortal like me even at 200mm, but 1/40 is already too slow for most of my human subjects.
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Ray Chen
http://www.arrayphoto.com