If you enjoy a good laugh, check out what pops up when you google "canon 6d review" - it's the busy bee "Mr. JPEG" Ken R0ckwell (if that guy exists at all and isn't a front for a clever team) that knows how search engines work, so no "hands on" here though it really is a "no hands on" review:
www. kenrockwell. com/canon/6d.htm
www. kenrockwell. com/canon/6d/vs-5d-mark-iii.htm
Actually he has a point when he states that the specs of the 6d are made to upsell the 5d3 to "rich people". But he seems to read this site because he enjoys contradicting some things that are eternal common knowledge here, so esp. the 5d vs 6d is worth checking out :-> ... just that the 5d isn't $3500 anymore, but he didn't notice that.
The 6D has a simpler AF system. You may prefer either; gearheads will prefer the 5D Mark III.
The Canon 6D adds an eight-way up/down/left/right control to the big rear control dial, and thus eliminates the need for the old thumb nubbin.
Top shutter and sync speeds are the same. No one uses 1/4,000, much less 1/8,000, so the 5D Mark III having 1/8,000 makes no difference in actual shooting.
Yes, there are many minor differences in the sensors and shutters as I'll outline below, but these are designed merely to help upsell innocent rich amateurs to the 5D Mark III; they aren't different enough for a real photographer or someone without a spare $1,400 lying around to blow on camera bodies to worry about.