I also had both, and I think that the difference is often not a big deal, but I like the 35 L better, it seems smoother.
And this has been my own experience as well, I think the Art 35 is a sharper Lens, and seems to have a better focus set up, but I still go back to the 35L as to me, the Bokeh is just smoother.
I think perhaps there's a lot of unrealistic hope surrounding what Sigma can/are able to do with competing lenses, My thoughts are that they are doing a pretty good job with the Art Series, the 35 is good and compares very well with it's target Market, in particular the Canon 35f/1.4 L, I've heard the new 18-35 f/1.8 is pretty good as well, but in general can Sigma produce a Lens at less than half the price as good as an Otus 55, or even a Zeiss 15f/2.8 ?? I'm not convinced, but it would be wonderful to see it if they could, I just don't think they can or will.
Canon made a pretty good Lens in the 50f/1.2 L, not perfect for sure but pretty good, if Sigma can drop a 50 f/1.4 Art with AF into the Market that solves some of the issues that the 50f/1.2 L has, it will sell and sell well especially with the Canon users, but the "aiming at the Otus" hype from Sigma ?? it's just that, marketing hype.