17-55 ef-s + 24-105, or 24-70 2.8?

[Kayaker72]

Looking for opinions- Currently have a 7D with the 17-55 2.8 EF-S lens, which I love. Didn't think I'd upgrade to full frame anytime soon, but the 6D is looking good, particularly for its low-light capabilities. I shoot mainly low light situations like concerts. So 2.8 is really about the slowest I can use for a lot of shows.
It seems a no-brainer to get the 6D bundled with the 24-105. Trying to decide if I should keep both lenses, and carry two bodies at shows, with fast primes on the 6D and the 17-55 on the 7D. OR sell both and buy the 24-70 2.8 (original). I think the 2.8 might be more important than IS. Anyone of experience with all these lenses?
Thanks!

More of a direct response to the original post, the focusing in low light of the 5DIII is much better than the 7D. Really, it is often the difference of getting focus in low light versus not getting focus in low light. And the 6D is supposed to be even better than the 5DIII in low light focusing (EV -3 sensitive vs EV -2 at the center point). Popular photography timed the 5DIII AF at EV -2 at 1.15 secs. I saw that the 6D review is on newstands, but it isn't online yet.

I have noticed better low light focusing on my 5DIII with the 50 f/1.4 compared to the 24-105 f/4. I assume this is because it activates the f/2.8 senstive AF points (I haven't checked the lists yet). So, if you already have the fast primes, I might go with that. Otherwise, I would probably be selling both lenses and getting f/2.8 (or better) glass on the 6D for your low light situations.

ISO performance will be the other thing that really impresses you. There is a thread over at CR posting high ISO photos taken with the 5DIII. If you believe DXO, the 6D should even be a bit better. From what I've seen, there is a 2-3 stop improvement over the 7D. I can clean up ISO 6400 images and I am beginning to think I can use ISO 12,800 images.