Submariner wrote in post #16699488
Wise move, I upgraded from a 7D.
Certainly would hate going back with autofocus or build quality.
Plus there are more sensitive AF points if you have F2.8 or less lenses.
Plus moving the FF means longer lenses and I think the weight and size of the 5D3 helps one wield say a 70-200F2.8L Mk II. ( sure others will disagree )
With the faster more accurate AF on the 6D over the 5D2 and 5D, this means that center point, which was already pretty good as long as you had the assist points turned on, is substantially better. It is easy to see how many, however abrasive, are passionate about their 6D doing everything they need it to do.
However, I have learned to really lean on the 1D AF system, and the 5D3 is very, very close to that. The 19 pts on the 7D was good, but having a wider range of AF points means handoff to the other points work even better, zone AF is very smooth at tracking, and the outer points on the 5D3, provided you use the right lenses, is as good or very close to the center point on other cameras. Less recomposure is good, and I can track a player along one side of the frame so that I don't have to crop the composure out later. I never use center point in either orientation during basketball shots, simply because I don't want the guys' butts in focus, I want their faces. 
I use very close to the top AF point for tracking in portrait mode (actually down and to the left a bit), so I can track the players movements. I can pick up a player as they go up for a 3pt shot too with this configuration.


(another fun one, worth framing)
The other great thing about 5D3 and 1DX shooters is that they don't belittle the 6D, they know it is a good camera, and in fact several have one as a back up FF.

Will the 6D satisfy the OP's needs? Yes absolutely. If they want more and have the budget for it, and want a camera they can grow into, then the 5D3 fits the bill.