Thanks for your review. I also moved from the T3i to the 6D and am very happy!I'd suggest trying back button focusing on the 6D. That way you can just hold the button down while you re-compose and not deal with re-focusing issues. Once I switched, I've been hooked.
I have heard how much better the 5DIII is but I could not justify spending the extra $1K for what I would get, especially since I am not making money with it now. After reading Justin Abbotts review I decided that the 6D was the better option for me.
I found that ISO 800 was the limit in the T3i, but use 3200 in the 6D with good photos. I shot an event without flash and posted it at [url]http://www.pbase.com/collink/art__soul]http://www.pbase.com/collink/art__soul] [url]http://www.pbase.com/collink/art__soul [/url] for examples of what the 6D can do in low light.
The autofocus is one area that folks have complained about. I am used to setting on one-shot and using the center point and recomposing on the T3i. On the 6D I find that rather than holding the focus it will refocus. I have gotten used to using the other points. Since I am primarily landscape/architecture this has not been a problem. I have taken good shots of go-cart racing in bright sunlight, but had to frame wide and crop.
I agree with the image quality. The amount of detail moving from the APS-C to full frame sensor is amazing. I also notice that I get much more flexibility in cropping.
The 6D is not the 5DIII. If you are not taking indoor sports then it is a better value.