To the original poster, did you really expect to be impressed by the 6D's autofocus? I considered renting one as well, but instead I just bought one, it will be here this week. But I'm not buying it because of its ability to AF at a world class level. However, I feel I can definitely make do with it. I tried a 6D in a store, and it had the 24-105 mounted to it. By comparison, I had rented a 1D Mark 4 last fall, along with a 24-105 lens (I also used the 1D4 with several of my other lenses). I found the focus speed to be slower than I thought it should be (but still faster than any rebel) with the 24-105, even on the 1D4. It didn't seem that much slower in the store on the 6D, if any. It even seemed at least adequate in servo mode, in the store. I was kind of surprised at that.
The 24-105 is just not the fastest AF lens, and the 50 f/1.8 is extremely slow at AF. The 24-105's focal range is why you use that lens...not its autofocus speed.
If you wanted to test ultimate AF speed of the 6D, you should at least try a really fast AF lens. MY 135 f/2 focuses about as fast as any lens I've ever tried or rented. Yet even it only focused barely faster on the 1D4, than on my 50D...and actually could not AF at all in low light on the 1D4, where my 50D actually could (at least with center point-only selected). I feel my 50D's ability to autofocus fast and accurate, just may be superior to all other Canon crop cameras and rebels, other than of course the 7D (and that especially includes the 60D).
I also agree with Nishi Drew above...but I'll take it one step farther. If you need the fastest AF speed, don't even get a 5D3. Pony up for the 1DX. In the future I will either buy one, or else its replacement. The 1DX is far superior in its ability to autofocus quickly and accurately, than every other camera on the planet, at least as of now.
The only thing I highly dislike about the 6D so far, just from trying it in the store, is the magnify button. I want to immediately zoom into 100% of the image to see if I got sharp focus, and not have to spin the top wheel. Hopefully there is a shortcut, or else someone will come up with one.