I am still wondering about that. I dont have much lenses for Full Frame and I feel the primes I have now ar not focussing as fast as my t3i+15-85mm. It's not perceptible for static objects but for exemple, with the t3i, I can walk fast in a crowd, the lens at my hip, fire away, and the AF will be near instant with the shutter press. I will get clear pictures. However I tried it with the 6D and the sigma 35mm to no avail. There's a pause between the shutter press, AF response and the actual shutter release. The subject is already away. You missed your shot.
I hope it's just the sigma lens or a difference from the t3i to 6D that I have yet to grasp. I need more testing.
Edit; but the 6D is the clear winner in low light of course. You can focus in moonlight really. Also there's a big difference in picture quality. Well worth the money.