as most people here have noted, everyone could pretty much see from the start that the 6D AF was a warmed-over 5D Mark II AF system. as a 5D Mark II owner, I have to agree that it is functional but really not "impressive", when it is pushed anywhere near its limits of low light or action (or heaven forbid, low light action) it really falls down flat. the other 90% of the time, it does it job fine.
whoever told you the 5D Mark III's AF system was hardly any different from the 6D was a total liar, they're really not in the same ballpark. if AF speed and accuracy matters to you, it's worth investing in the 5D III.