so where it's this in the packing order of the Canon 70-200 is f2.8L mkii. better, worse, our just a different lens so the comparisons aren't really fair.
Well, judging from TDP's IQ charts for the IS II and the previous version of the sigma 120-300mm f/2.8 OS, the canon is way better (at any aperture) in the 70-119mm range, and sharper across the frame @ f/2.8 from 120-200mm, but it just doesn't compare at 201-300mm. Throw a 1.4x III on the IS II and @280mm the canon is sharper from mid-frame into the corners @ f/4, but the sigma is sharper in the center. And the sigma also does f/2.8 @ 300mm - so it's got that going for it, which is nice.
Honestly, I wouldn't be surprised if the v2 sigma has better IQ than the previous edition, if for no other reason than that it seems like sigma's finally taking quality control more seriously.