Click on the link below for a great photo and video comparison between Canon’s 5D mark iii and 6D cameras.

Aside from low-light focus (which is quite a significant variable), it appears as though the 5D3 has the edge over the 6d. Burst rate, anti-aliasing, moire, noise at low to mid-range ISO, etc all favor the 5D3. Having said that, the 6D is about 1/2 the price of a 5D3. Not bad for an entry level full-framed sensor.  Dave makes an interesting point during the intro as to the need for Canon to start paying more attention to dynamic range instead of ISO.  While the ability to produce low noise images when shooting in the dead of night under Navy SEAL night-vision conditions is an important criteria, the photography community at large might be equally well served by having better dynamic range in their images.  Then again, what do I know.  I decided  a long time ago to light my subjects with more than one candela of light.

http://www.learningdslrvideo.com/canon-6d-vs-canon-5d-mark-iii/