If you don't do stuff like this:
Red-tail hawk flies by by
dcstep, on Flickr
or sports, then I think that the 6D is the preferable body for most people. The 5D MkIII's tougher build quality may be important to some, but I think the AF system's ability to accurately acquire focus and keep focus on moving subjects is the reason to spring for the more costly camera. Most people never really put that to use. The 6D can track the kiddos and manage most situations quite well.
Dave