Nick Aufiero wrote in post #18301909
......... In a world ruled by technology I can't be bottlenecked by old stuff
This statement right here made me laugh out loud. The client is not going to be able to tell what you shot the final image with. There are shooters out there still doing paying work with film. Meanwhile, your average medium format camera is far less technically capable. Don't get caught in the technology upgrade cycle, you'll never be happy that way. Besides, there are things the Sony is "behind the technology curve" on. Camera control software being one of them.
Meanwhile, as someone who shoots both Canon and Sony I can relate to your dilemma. I kept my 1Dx because I prefer the ergonomics of it, and the AF on the Sony becomes less than worthless in light levels that don't phase the Canon. I still prefer the color I get from the Canon, and find the Sony can sometimes present a real challenge to get good color from if you don't make a profile. The ergonomics of the Sony can get real frustrating when you're trying to shoot fast, as that camera body is really cramped, and desperately needs a joystick. The sensor for the EVF is too sensitive as well, cutting off the rear screen when you're trying to chimp.
I rented an A7RII from Lens Rentals before I bought mine (Jan. 2015). I constantly cussed the ergonomics of it, was endlessly frustrated with the AF (using my adapted Canon glass), and found using it to be a generally unpleasant experience. I didn't like the lack of third party software and equipment support, and was leary of Sonys poor reputation with service and repair. Of course, when I started playing with the files in the computer, all of that went away. This camera is the medium format back that I wanted but couldn't afford. I use this almost exclusively when shooting in controlled situations. My Canon has been relegated to family travel and events.
Sensor heat. Not an issue. Don't sweat it. It's yet to be an issue for me, including shooting in studio where the camera is powered up long enough to wear the battery down while I adjust the lighting and set.
Personally, I think if your primary shooting is portrait and headshots you can't go wrong with this camera, but you'll definitely need native lenses. Just know that the AF will fail in light that the Canon wouldn't have flinched at. Having native glass will help, and other than low light, the Canon AF on the 6D will be worse or the same under normal light levels. Just know the ergos are going to fight you. It is definitely small and cramped, but for portrait shooting it's not an issue. . . . . and be sure to buy extra batteries. 
