You'll find sharpness wide open the same but it would pretty much come down to a few specific things per lens.
35mm IS - Lighter, IS, better AF and slightly cheaper.
35mm Art - A stop extra light, less vignette wide open.
I love my 35 IS and if I was going to trade it for another 35mm on a Canon body, then it would be the 35L II. Although if you have a copy of the Art that has no issues with wide open performance and AF, I would recommend keeping it. Sigma make some really great lenses and if you do sell it and decide to buy one back, you may very likely not get as good of a copy the second time. I am hesitant to sell my Sigma 85mm F/1.4 EX for that reason, I may not find another copy that works as well as this one.
Actually the 35 1/4 does not give you an extra stop of light. DXO tests light transmission and utilizes a tStop value for comparisons. Testing shows that the Sigma 35mm f/1.4 ART has a tStop of 1.5 on full frame Canon bodies and 1.7 on crop bodies, while the Canon 35mm f/2 IS USM has a tStop of 2 on all bodies . So on a full frame body, the Sigma allows in a bit less than 1 additional stop of light and on Canon crop frame bodies it lets in a bit more than 1/3 stop of extra light.