However, Canon and Nikon both show it off better, and so when I’m looking for a 24-70mm lens to use as my next all around lens, I’m opting for the Canon 24-70mm f/2.8L II over the Sigma 24-70mm f/2.8 Art Series.
IS has its price in this focal length as it seems. Nikkor seems to struggle as well, at longer FL:
http://www.photozone.de/nikon_ff/1010-nikkor2470f28vrfx?start=1
Not to mention, it's noticeably heavier and more expensive.
Many people criticize Canon not to include IS in the 24-70/2.8L II. Seeing these results with stabilized 24-70 lenses, I'm glad they did omit IS altogether...