The 5D and 30D shared about the same AF system. The 6D has a better overall AF system. How much better?
Taken from various online reviews...
The bottom line is that the EOS 6D's phase detection autofocus system is about identical to that in the 5D Mark II, at least in terms of center point accuracy. It may be worlds better in low light, Servo mode, or with peripheral sensors; I can’t comment on that. And even if it doesn't rival that of the 5D Mark III, I actually didn’t find the EOS 5D Mark II's center point autofocus bad at all.
Probably the biggest feature difference between the Canon EOS 6D and the 5D or 1D-family bodies is autofocus. The Canon 6D’s 11-point AF system also offers the strongest low light performance of any DSLR on the market. With focusing down to EV-3, the equivalent of moonlight, subjects remain in-focus in even the most challenging lighting conditions – offering the freedom to shoot landscapes or portraits at night and capture the true atmosphere of the scene with minimal noise. Compare this to the -2 on the 5D Mark III, and the -0.5 on the 5D Mark II. This is a clear win for me as I often use ND filters (up to 10-stops of light) to push exposure times and occasionally rely on autofocus bracketing to obtain critical sharpness. It accurately and predictably locks on subjects even if the viewfinder is black. Amazing
No reason to beat around the bush: this has been one of the Canon 6D’s biggest questions marks since it offers a much less comprehensive autofocus system than either the Canon 5D Mark III or the D600. Canon has touted its sensitive center cross-type point, which is rated for use in darker environments than the 5D Mark III’s, as a key differentiator between this and competitors, including the older but still available 5D Mark II, and for good reason.
The center point is indeed a very impressive, responsive and fast AF point, as tested in a number of environments and lighting situations, with a range of lenses. It hardly ever hunts for focus, even when going from infinite to something quite close. Canon has done a very good job here, and that’s likely perfect for anyone who generally uses the focus and recompose method of taking pictures anyway. But those side points are a different story.
First, they’re distributed in a fairly tight grouping towards the middle of the frame, so even when you select outer points you’re not getting all that far away from center. Second, they’re a far cry from the accurate middle point, and especially in low light situations, you’ll find them hunting quite a bit, and often coming up short, unless you’re pointing at a very high contrast subject. I’d hesitate to say the outer AF points are a noticeable improvement from even those on the T3i, in fact. The good news is that with an external flash attached that carries an AF assist beam, focus limitations are mitigated considerably. The bad news is, you won’t get the outer reach of either the 5D Mark III or the D600.
When you leave your hand and to grab onto your vertical grip, you will lose time. 
and my 
