Photozone.de has full test results for both of these lenses (on 5D Mk. 2). It's evident from the resolution numbers that both lenses were optimized for center sharpness at large apertures. Neither one does particularly well in the border/corners, but I was surprised that the Sigmalux was able to edge out the 50L in resolution, vignetting at large apertures, and chromatic aberration (the latter by quite a bit).
I realize that there are many other factors that determine the ultimate image quality of a lens (for example, micro-contrast), so I'm not trying to start a flamewar by claiming that one lens is inherently better than the other. The Sigma's performance in the 3 areas below is definitely very impressive though, especially considering that you can buy one for 1/3 the price of a 50L.
Canon 50L on left, Sigma 50 on right:
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