I have been following the 50A since it was announced and here's the differences that I'm aware of big & small:
Canon = more expensive / Sigma = less expensive
Canon = a Canon [and red ring] - important to some / Sigma = a 3rd party lens (with the USB dock, not a big concern)
Canon = weather sealed / Sigma = not
Canon = small & stubby / Sigma = big & heavier (24-70 Mk I size)
Canon = USM, but some backfocus at MDF / Sigma = the jury is still out, but the USB dock and AFMA are recommended
Canon = center IQ good, corners soft (on test charts) / Sigma = Zeiss Otus 55 sharp
Canon = field curvature wide open, low distortion / Sigma = flat field, nearly zero distortion
Canon = you like vignetting, right? / Sigma = medium vignetting
Canon = great color / Sigma = great color
Canon = good contrast / Sigma = more contrast
Canon & Sigma = good flare resistance
Canon = CA not great / Sigma = good but not perfect CA control
Canon = LoCA ugly / Sigma = LoCA not Otus, but pretty good
Canon = a bit less contrast in bokeh / Sigma contrasty bokeh
Canon = larger physical aperture for bigger OOF elements / Sigma = no slouch, but slightly smaller OOF blobs
Canon = 72mm filters / Sigma 77mm filters
Summary: both are great lenses and it's a good time to shoot 50mm
The key differences are price, size, weather sealing, and requirements for edge-to-edge sharpness. AF accuracy may be added to the list, but it's too soon to tell.
For me, the 50L's small size is a big plus and since I use it for portraits 99% of the time, edge-to-edge sharpness isn't critical. If I were in the market for a 50mm lens, now, though, I'm not sure what I'd do