Here's a comparison (with the 70-200 Mk I, at least) I found that may be of interest:
Canon 135mm f/2 vs Canon 70-200 I L Lens Review / Comparison Test
I think the differences are pretty subtle and with the Mk II (he also did a Mk I vs. Mk II comparison), I'm sure they are much more subtle. The 1-stop for speed is by far the biggest difference.
thanks Mack. subtle indeed! i would guess that 9 out of 10 people wouldn't notice the difference and wouldn't have a preference between the photos.
A matter of opinion. In the second and third comparison series of images, the difference really isn't all that subtle at all. The 135 at f/2, is just quite noticeably smoother in its bokeh, and also throws that de-focussed background into relief where the highlight details appear 40 to 50% larger. The difference, is going from f/2.0, to f/2.8...and the bokeh is slightly less smooth...that's all.
I'll grant you that the first set of comparison images, is more similar. But with subject distance the difference is going to decrease, because the background becomes closer to being in focus anyway. And again, the real reason most users think the 70-200 f/2.8 can have very good bokeh, is because they are using it at focal lengths longer than 135mm, at f/2.8...where the depth of field is that much more shallow...but also the angle of view is more narrow.