Part of producing great IQ is auto focus accuracy. The 5D3's AF accuracy is overall, superior. The 6D is also worse in flash sync speed. The 6D's IQ is not better than the 5D3's by any measureable amount. So...
I cannot think of any reason that if you already had a 5D3, you'd gain anything by selling that, and getting a 6D. Starting with the 6D is a great decision, because it's a great camera. But even for things that are great, there is always something better. Going from a 5D3 to a 6D would be a net loss.
My ability to get it done was definitely made easier with the 5D. I also agree that the 5D taken as a whole is a better camera. The AF system is head and shoulders above the 6D. However, as discussed before, it was move which allowed me to maintain similar IQ and put money back in my pocket. I didn't feel like I needed everything the 5D had to offer at the moment. And doing plenty of work with the 6D since has proven that to be generally true (albeit with less ease).
Some say the IQ of the 6D is better. I cannot agree with that. What I can say though is that I do prefer the raw files in post.
Flash sync speed difference is a non point as the difference is beyond negligible in real world use. I'd say the max shutter speed difference would be more of an issue to mention that the sync speed.
Yes, the 6D has been a downgrade. But it has been one that has worked for me regardless. Definitely looking forward to the day the new pro ff bodies are out though.
Again, the reasons I made the move has been documented extensively in this thread along with all my other discoveries. And at this point, I still believe I made the right call.
Again, I do not believe the 6D is > 5D3. In case anyone is unsure of my view on that comparison alone.