I've got a 70-200 f/4 IS and its an excellent lens. However, since picking up a 135 f/2, I rarely use the 70-200. The 135 is sharp at f/2. While the zoom is good at f/4, it is much better at f/5.6. All things being equal, you can use a much faster shutter speed with the 135, whereas a slower shutter speed on the zoom might lead to some motion blur. The wider aperture of the 135 also allows you to blur backgrounds more if you want (which is a nice look for some portraits). Also, its not a big white lens - you feel a little less conspicuous carrying it. They're both around the same price - I'd at least give it some thought.