Only a little bit better? For how much less $$? Isn't a 5DIII $3400? If i could afford that, I would have bought a 5DIII. I cannot, so I am VERY pleased to have the latest FF sensor, with great noise handling, for what I can afford.
My point, even if the 5DIII is $2990, the 6D is still cheaper, has the latest FF sensor, and other options the 5DIII does not have.
Which is better depends on what one wants and what one wants to pay.