AF speed is nearly identical. The MkII will intially go faster, but will hunt toward the end. The Tamron will go a touch slower, but does not hunt. Mind you, this happens in lower light condition. In bright sunlight, I went back and forth between focusing on a far away object, and then one that's only a few feet away. The difference is so slight, that I doubt I could tell the difference if I didn't already know what I'm holding in my hand.
For me, the limitation on tracking subject will be the 6D's own AF system. Not bad in bright light, but will be subject in lower light (unless you use center point all the time).
Interesting comparison! My theory about the 6D's AF failing too often, is because when you leave all points active, for some reason the system almost never hands off to the center point. It keeps trying to focus with the others even when the subject is clearly under the center point. It has no problem handing off amongst the other outer points...but if it could only give equal weight to the center when all are active (rather than neglect it), I feel it wouldn't lose lock so easily. If Canon would ever change this via firmware or something, that would be good.