Well I bit the bullet and decided to go with the 6D and not wait for the 70D.
I believe the AF and FPS will be fine for the sorts of "action" shots I need (i.e kids running around playing sports outdoors)..
A co-worker of mine purchased a 6D. He was showing me some of his action shots (soccer games, BIFs including hummingbirds). The 6D did not seem to have any problem with auto focusing.
The 6D can be made to lose tracking in servo mode, especially on subjects with lower contrast, or with a difficult background or foreground (also depends on which lens is attached...some will make the problem worse). As available light gets darker, things don't improve. But the problem with comparing how much worse this is than with other more capable bodies, is that these problems also happen with the 5D3 and 1DX. They just happen less often, generally (and up to that point where the 6D's center point can still achieve fast focus lock in "single shot" mode...where the 1DX and 5D3 are left hunting forever.)In my opinion, for 90% of the type of photography the general public does, the 6D's autofocus is capable of getting a very high percentage of their shots in focus...even sharp focus.
It really is those who are experienced with the 5D3 and 1DX, who are able to easily find fault with the 6D's autofocus ability. These people would never be caught dead using a 6D for anything other than long exposure tripod photography, because otherwise, word might get back to their buddies that they were using the inferior camera for "critical action events". That just will not do.
If you happen to own one of the "great white" supertelephoto lenses, you are already very serious about getting high quality results. If you shoot events with this lens, that are highly speed intensive, occurring in moderate to good light, then the 6D is not the right choice at all
. If you mainly shoot wildlife that is either slow moving, or large, or else small and stationary...in low light with this great white lens, then the 6D can quite possibly be the best choice
. The 1DX has a lot less color (chrominance noise) than both the 6D and the 5D3, so if you can tolerate its loud shutter, mass, expense...and you can make it autofocus in the above low light situation...then it is the best choice (currently...its successor might more likely the best choice of all).