From The Digital Picture, it looks like the Sigma is sharper and has less vignetting at 1.4, but the Canon has more accurate autofocus. I returned the 24L II due to inconsistent AF on my 6Ds. as for the sigma art, I have 24-35, 20, and 35 - all calibrated with USB docking and are very accurate on my 5DSr and 6D.
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