My DSLR upgrade path was XT -> 60D -> 7D -> 5D3 -> and another 5D3. I've kept them all -- in part due to kids with an interest in photography. The kids get to use the 60D and 7D.
The 60D is very similar to the 6D in AF specs. I added the 7D because I shoot mostly indoor sports and found that the 60D had some issues with fast moving erratic subjects, like figure skating. The 7D was noticeably better at tracking sports. Does this mean that the 60D can't shoot sports? No. But, I shoot enough sports where the higher keeper rate from the 7D is valuable to me.
This is the decision that you need to make. Do you shoot enough action where the 6D AF system will hold you back?
Now let's add another dimension to the thought process. I was thrilled with the 7D/70-200 f2.8L II combo. Indoor sports was phenomenally better than the XT/70-300 4.0-5.6 that I was using. Then the kids that I was photographing got better at their sport and I needed faster shutter speeds. This meant higher ISO -- 3200 and up. This also meant a lot of post production noise cleanup.
I was definitely in the "crop body is all I need" camp. I prefer to use two bodies to avoid changing lenses and thought that I had the dream system -- a 60D/17-55 f2.8 and a 7D/70-200 f2.8L II. I didn't want to believe that FF was really that good. I didn't want to pay for it. Then I read more from those who switched (mostly on CR). Ultimately, I realized that the low-light limits of the 60D and 7D where holding me back and I took the plunge with a 5D3.
Is there much of a difference between FF and crop. YES. That's why I added a second 5D3.
Fortunately, the 5D3 has a better AF system than the 7D. The only downside for me was a slightly slower FPS and buffer rate for burst mode. But, I use burst sparingly.
To be fair. The 7D did a great job with adequate light, including portraits. Curiously, I've seen many mass production portrait photographers, the kind hired by schools, use older crop bodies for their work. So, it's not fair to say that the 7D or the 7D2 isn't good for portraits.
But, when you compare a FF body to a crop body, there is a huge difference. When my 5D3 arrived, I took test shots against the 7D around the house. The first thing jumped out at me was the greater color depth of the FF body. Images looked more alive. Then, of course, the low light performance that I hoped for was better than I imagined. The 5D3 has been fantastic in low light situations. The 6D is supposed to be better (if action isn't an issue).
Another big benefit of FF is that the 70-200 is much more useful focal range for indoor events, indoor sports, and portraits. (I love the 70-200 2.8 on FF for portraits.) FF is also sharper. With ISO 800 and up, my quick tests show that a cropped FF 200 mm image is sharper than a full crop body 200 mm image. However, I still grab the 7D for outdoor sports -- soccer and baseball -- just for the extra reach.
I'm intrigued by the 7D2, mostly with the 10 FPS and the video AF. Still, if I had to choose between the 6D and the 7D2, the 6D would be my first choice. The big question is how you value that AF and FPS performance of the 7D2. In my case, with thousands of indoor action shots, I may struggle with the AF of the 6D. I would have to rely more on my timing skills to get the shot. But, I need the shutter speed that this high ISO of the 6D offers. For indoor available light, the FF sensor holds more value to me than the 7D2's AF system.
Others have been more succinct than I -- 7D2 for outdoor sports and wildlife, 6D for everything else.
I should point out that the 5D3 offers the best of both cameras with little compromise, save for a few hundred extra dollars. But, deals on 5D3 bodies are becoming more frequent.
