So far, I'm finding the Sigma 35mm very impressive. In addition to great image quality, the focus is quick and spot-on. No front or back focus issues like I've seen with the Sigma 50mm. And shooting in the dark, there was no focus hunting, even using the outer AF points of the 6D. The build quality is fantastic - reminds me a bit of Zeiss lenses. The Sigma 35mm is in a completely separate world compared to the Sigma 50mm. Be aware though - because of that build quality (and metal inside the lens), the lens is a little heavier than the Canon 35mm f/1.4. However, the lens balances nicely on the 6D, and feels solid in hand.
Thanks for the info - definitely looking into it. I'm not worried about heft - the TS-E 17 weights 2 and a half pounds!