In terms of image quality and out-of-focus rendering or (also?) “bokeh”, does one of these lenses have an edge for portraits and head shots at 100mm?
70-200mm f/2.8L IS II (8 rounded aperture blades)
100mm f/2.8L Macro IS (9 rounded aperture blades)
If there is no substantial/significant/notable/etc. difference between the two lenses at 100mm, I would be inclined to use the 100mm macro on a shoot because it is lighter.
However, if the 70-200mm lens (at 100mm) has some sort of appreciable advantage over the 100mm macro lens, then extra weight would be unimportant to me, and I would use the bigger lens.
From what I believe I have read elsewhere in Canon Rumors , but am unable to find exactly, is that the macro lens is not recommended generally for portraits (?), and that the 70-200mm is preferred, along with primes such as the 135mm f/2L or the 85mm f/1.2L.
Might anyone share thoughts or insight regarding this question?
Here are my thoughts: first, you should loosen up your perception of what a "portrait" lens is.
I have the 85mm f/1.2 lens, usually thought to be the near-perfect lens for portraits in the Canon lens lineup. It is wonderful, but I take a minority of my portrait assignments with this marvelous beast. One can take a great portrait with almost any lens, from ultra-wide (fisheye to 24mm) "environmental" portraits, to "normal" 35mm to 60mm lens "medium" shots, to super-tight, very long focal length portraits, with lenses up to even the 300-400 mm range. Think creatively. It's not that a 100mm lens would not produce extremely pleasing head and shoulder - and even tighter head-only shots; it's just that it would be preferable to have more focal length options, especially in the 70-200mm range which covers most of the best focal lengths for people shooting. That is why you should strongly consider the f/2.8 zoom, in addition to, or instead of the 100mm macro. It also affords you the cropping and perspective that you might prefer when you can't necessarily change the distance between you and your subject, all things for you to consider.
As to bokeh, well, as in all high contrast zooms like the great 70-200 mm f/2.8 IS II, bokeh is a little busy compared to many simpler single focal length lenses, but it is still very good compared to most zooms and is usually more than acceptable for narrow aperture portraiture. Significantly better bokeh than the zoom would require the 85mm f/1.2, the 135 f/2.0, the 200mm f/2.0, or, perhaps, yes, maybe (by a hair) the 100mm 2.8 macro.
Good luck finding what you want and using it for what you need.