While it is true you are more limited with your AF points on the 6D, keep in mind you can focus and crop instead of recompose.
Certainly, but I'd consider it a a workaround in the extreme.
Well, considering that the 5D3 AF points definitely are far from filling the entire frame, it is a workaround you will have to employ on the 5D3 as well - perhaps just less frequently. At f/1.2, one should never focus and recompose even a smidgen as you well know due to the thin DOF.
More importantly, the 6D allows for easy switch to Eg-S focus screen unlike the 5D3 - so IMO 6D actually has a significant advantage for manually focusing the f/1.2 lenses and seeing their true DOF in the viewfinder.
Yes, the focusing screen can be installed (I have the Eg-S screen) but I'm specifically commenting on the AF which doesn't work too well with very large aperture lenses. Comparing the 5D3 AF with the 6D MF with Eg-S screen is like comparing apples to oranges. The 5D3 outer AF points will lock on with superb precision and you would have already taken the shot and moved on to another in the same time you would be focusing manually with the 6D.
Its not really comparing apples with oranges because the Eg-S allows you to see in the viewfinder more accurate DOF - even when autofocusing - than the 5D3's stock screen which is not user replaceable. 5D3's fixed focus screen only shows about f/2.8 DOF while the Eg-S on the 6D will come close to the DOF of f/1.2.
So, the fact that the 6D allows the user to swap to Eg-S means that what you see in the viewfinder when both AF *and* MF will more accurately represent the end product. If you do choose to MF to that end product, it is worlds easier on the 6D using Eg-S vs the 5D3's fixed screen.
Yes, the 5D3 will likely allow to set up your shots faster, but that does not make the 6D AF useless.
One could argue you are looking for trouble in general if you try to autofocus at f/1.2 all the time no matter what camera you use.
All EOS cameras AF with the lenses wide open, so a f/1.2 lens will AF wide open at f/1.2 no matter what settings you use to shoot.
Correct, when aperture is set to f/1.2 DOF is so thin that the very slightest difference between where you thought you were focusing and where the actual camera focused will cause loss of detail. MF with Eg-S/matte screen allows you to easily see if you got the shot or not before you take it - and you don't even have to go into Live View.
The cross type outer AF points work extremely well on the 5D3. See below photo taken with a 5D3 with 85L II - 100% crop of the AF area also provided - I have been using this combo for quite a while now and am happy with the results.
Not saying that 6D is a bad camera, but the whinging that takes place over the AF happens deservedly so.
Edit: I'm not sure how to post the 100% crop. The 100% crop image that I can view here on CR is larger than what I see in LR with a 1:1 view - any tips on how to post the exact image?
I think the whining about the 6D AF depends of what you take shots of. Birds, sure, the 6D AF probably not good for that. But for shooting people, the 6D AF is superb. Finally when one wants to MF (or see DOF when AF in viewfinder below f/2.
, as one often may want to with an f/1.2 DOF, the 6D actually is better than the 5D3 IMO.
If the 5D3 had a user replaceable focus screen like the 6D (or a stock screen similar to the Eg-s) I'd be quicker to throw the 6D under the bus, but the 5D3 does not, which is certainly a large demerit against the 5D3 when AF or MF in the viewfinder with fast lenses at apertures wider than f/2. 8.