I've been using my 7D for a couple years, but have been dying to get a full frame for a while. I debated selling the 7D and getting a 5DIII, but decided to keep the 7D and pick up a 6D. I can use them both with my 24-105L and 70-300L lenses. I love the reach and speed of the 7D for birds/sports.
On my 7D I've used auto ISO with a maximum of 800. What would you suggest is a comparable maximum ISO for the 6D?
I own a 5d3, and have rented a 6d. ISO on both are very close. Everyone has their own threshold for noise/usable ISO - so that really depends on what you shoot. I do know that on either a 6d or a 5d3, I don't worry if I have to use 6400 and up... (the quality of shots I see at 8000, and 10,000, even 12,800 are quite frankly outstanding).
I used a 7d for about 3 years prior to snagging the mk3 and I'd push the 7d ISO to 4000 in a pinch. More times than not though upon first look i would get worried that the focus was off, or, that I was not steady in the shot...then I'd zoom in on faces...focus was fine, it just looked like the focus was off because of the noise - also the images just looked so soft, again, due to noise. After getting the mk3, I tried using both bodies and found that I would only use the 7d if I could run below ISO 1600, which meant it was only useful for a small portion of weddings (either outdoor ceremonies or the parts of the wedding that occurred outside. So I sold the 7d and am now looking at a 6d as a very viable backup body.
I'd say play around, shoot in as many lighting conditions as you can and find your threshold for whats usable and whats not.