Thanks for the excellent responses and example photos.
I don't think I can make a bad decision here as both the 100L macro and 135L can take awesome portraits as illustrated by the samples and links provided. This is going to be a tough call!
I just re-read your original post.
First, since you've got the 70-200 f/2.8 II, there are only a very few reasons you'd want anything else -- and image quality is not one of those reasons
. Nothing is going to beat the IQ of that lens.
The 100L can do macro photography that the 70-200 can't.
The 135L is one stop faster than the 70-200. Big whoop. The 70-200 beats it in image quality, though the 135 is certainly no slouch.
Both are smaller and lighter -- and that's the reason you're indicating you're considering either.
So, here're my updated recommendations.
First, if macro photography is something you want to get into, the discussion is over: get the 100L.
But, if not, if macro is just a "well, I'd play with it if I had it" sort of thing, get some gaffer's tape and fix the 70-200 at 100mm. Shoot a full session with it, however you would if you had the 100L. Then do the same thing with the lens taped at 135mm. If you discover that you have a preference between the two focal lengths, that answers the question.
If your shooting doesn't reveal a strong favorite, run one of those EXIF analysis tools over the shots you've already taken with the 70-200 to see if a definite pattern emerges that way. I'd only recommend this after doing the gaffer's tape bit because this is an emotional decision as much as anything, and it could be that you want to use the one focal length but some sort of restriction forced you to use the other more.
If you still don't see a winner at that point...get the 135 f/2.8 with soft focus. It's very small, it's lightweight, it's cheap, it's pretty decent optically (though certainly not up to the standards of the 70-200 f/2.8 II!), and it's got a built-in soft focus filter for you to play with. Your primary expressed desire, after all, is for something small and lightweight when you don't want to haul around the big guns, and the 135SF fits the bill perfectly.
Also worth considering is its close cousin, the 100 f/2.0. It's even smaller and lighter than the 135 and one stop faster.