100L or 135L?

I dunno, I've kept my T1i and 55-250IS mostly because there are days when the 1D and big whitey 100-400L just isn't appropriate.

My 100-400L shoots at 135mm at f/5 so I definitely use the 135L at f/2 for low light situations. Believe it or not there are times when f/2.8 just isn't fast enough. Just one stop you say but that can easily be the difference between blurry shots at 1/200th and decent (though not necessarily great) stopped action shots at 1/400th. For some situations faster is always better.

For an emphasis on isolation, the 135 should be your lens, but I'm a 100L fan. F/2.8 is a tually a really good aperture for portraits, and it's just such a versatile lens. Mine is going to Canon to fix a recent problem with the focus ring, but I'm still going to miss it for the week or so it's gone. It's just such a great lens.

If you don't plan on doing much macro then get the the 135L, otherwise the macro.
IS is usually 3-4 stops