The new Canon 35L mkii is not just the best 35mm on Canon but arguably best 35mm ever made, Zeiss included, optically it is the best. It's built quality inside too is one of those that is "over engineered". The down side is that it comes at a price, 3x the price of the Sigma, it is not 3x better than the Sigma but it is better.
As for the 6D, no point really pushing other cameras now as it's been bought, if it does what you need doing then it's all that matters. (Although I would have opted for the Sony A7Sii, the low light video performance of that I believe is still class leading)
Read this below on how it's built inside.
https://wordpress.lensrentals.com/blog/2015/12/canon-35mm-f1-4-mk-ii-teardown/