The Sigma 50 f1.4 is definately a better lens than the Canon 50 f1.4
However, i did NOT get the Sigma 50 over the Canon for 1 reason, the 5D3 has in built CA correction for the Canon 50.
I know you can adjust it PP in photoshop or Raw or Lightroom etc…many way to do it..BUT i only use the 50mm for general shooting in jpg, and for those photos, i do not want to edit every single shot, just SOOC..so the in built correction on the 5D3 works brilliantly on the 5D3.
I do however own the Sigma 85 and sold my Canon 85 because i use the 85 specifically for commercial portraiture and i will edit every photo in RAW so the in built correction is not important for me in this case.
That's how i made my decision with the Canon 50 f1.4 and no i did not consider the 50L because i don't use the 50 very often at all