I love my Sigma 35 Art. The thing is a guaranteed focus machine, even wide open, and Since I bought it 7-8 months ago it has been on my camera almost constantly. I did a puddly little +2 AFMA but it really wasn't crying out for it to begin with. And of course it is so amazingly sharp.
Also, the colors are so vivid I almost can't believe it.
It feels very solid. Really solid, and high-quality. More solid-feeling than my 24-105L, which is plasticky (though in a solid package, same way a Corvette is plasticky fiberglass). Almost as solid as my 70-200 2.8 L Mk 2, and some of the difference there may simply be the fact it isn't so dang heavy!
Get what you are comfortable with, though I do not see the burningly-important point in IS in a short lens like a 35mm. If you are shooting people you can't really do that a lot slower than its theoretical reciprocal shutter(1/35th) anyway, because the subject's motion blur will kill you. I cant shoot my elementary school kids below like 1/60th, and they are not even all that squirmy. And if you are shooting landscape in dim light at 1 second, you should just bring a tripod, or rest the camera on a rock or something----gheeesh.