I was sent this article from my friend Gary Simmons... instead of rewriting and doing my own research, I figured I would just pass this article on to you.
Astrophotography enthusiast Don Marcotte did such a great job comparing these two cameras, I could not have said it better than myself... check out these two links. I was blown away at how well the 6D performed against the 5D MKIII.
http://www.petapixel.com/2012/12/13/canon-6d-and-5dmk3-noise-comparison-for-high-iso-long-exposures/
http://blog.planet5d.com/2012/12/the-high-iso-battle-of-the-full-frame-heavy-weights-canon-6d-vs-canon-5d-mark-iii/
