I had a poor copy of the 35 Art, but I know now they are not all like that, and calibrated with the docking they are very good.
I bought the 50 Art and I had also dismissed Sigma, but the 50 Art is a staggering good lens, and is now my all time favorite lens. And I have owned 90% of the lenses Canon has ever made in EF mount.
Get the 35 Art with the docking, you'll be much happier with money left over for the 50 Art as well. The 35 L II will NOT be cheap.
The Siggy 35 1.4 Art is amazing, and I've used everything.