With Phase Detection Auto Focus it is ultimately about how accurate the system is. To put it another way, it is all about how often the the system is accurate enough. Every system will have variation from one shot to the next. The question is what is the average difference from sharpest focus for each shot.
The best single place I've seen describe the differences between various Canon bodies in terms of AF is a blog entry by Roger Cicala at lensrentals.com. The 6D wasn't out when he did his data collection, but I'd be surprised if it was much better than the T4i in this respect. With a current generation lens design, he found that standard deviation for the recent 1D series bodies and the 5DIII was about one half that of the other recent Canon bodies.
Please note that the lens plays just as large a role in AF accuracy as the body does. The weakest of the two components will determine the best each combination will be able to perform. A 5DIII or even a 1D X will do little better than a T3 if a lens like the much maligned EF 75-300mm f/4-5.6 is mounted on it.