As others said, both are great lenses. I have both and love them both.
If I have to choose one, then I have to stick with 135L. For indoor portraits, 135mm is a little long. But the extra stop is enormously helpful. I have done some indoor portrait shots under normal light conditions. Shots produced by 135L wide open are excellent. The 2.8 aperture would not work as good as 2.
I rarely use flash. Indoor under normal light 135L wide open on 6D can produce sharp images with ISO 1600, 2000, or 3200. With 2.8, ISO would have to bump to 4000, 6400, or sometimes even high. Granted, 6D can produce nice images even with 12800 ISO sometimes. But when cropping, higher ISO images would not cut well.