I will also add +1 on the Sigma 50mm/1.4 (and I don't even sell it).
When I shopped for a 50mm, I considered Canon 50/1.4 and Sigma 50/1.4.
If you can afford the 50/1.2, great. But not in my budget.
Problem with the Sigma was the auto focus (which I use a lot). Very inconsistent, not reliable enough. But the copy I had seemed sharp, even at 1.4 and the out of focus rendering was what I was looking for. But frustrated at the focus issue (these are well known with the Sigmas).
I tried the Canon 50/1.4 and was not impressed at all by the quality wide open and near wide open. Could have been a bad copy. But I figured if I'm going to buy multiple copies to get one I like, I might as well try the Sigma again (my first choice).
So I did, I bought 3 at once, kept one returned the others. The one I kept focused close enough to the Canon USM and the sharpness was - even at 1.4 - completely usable. I normally am dealing with 40MP/60MP digital backs with sharp lenses and pixel peeping, etc, so I know sharp when I see it.
So I now happily own a Sigma 50/1.4. The sample below is at f4, but f1.4 is only a hair less sharp. Note the round points off the taxi lights.
I would say regardless of what you buy, I would purchase multiples, try them, keep the best. I don't sell Sigma, I sell Canon, so this is just from me to you, you owe it to yourself to try the Sigma.
Steve Hendrix