I've only used the two Canons, not the Sigma, but I've read a lot about it and looked at photos.


Autofocus:


The f/1.2 has the focus shift issue, and the Sigma has a poor reputation for QA, so I'd give the nod to the EF 50mm f/1.4 on that one. (The AF has been quick and accurate on my copy, at least.)





Durability: the Canons f/1.4 are right out on this category. The Canon 50mm f/1.4 is one of the most fragile lenses that Canon makes. Posts about focus repairs for this lens are so common that they clog the intertubes. I dropped mine several inches onto a padded carpet (with no camear attached), and it died. Lensrentals kindly shares their repair data that shows 19% for the 50mm f/1.4 and 15% for the f/1.2:





http://www.lensrentals.com/news/2010.09.20/lens-repair-data-40





That has improved a lot this year, so perhaps they fixed the problem:





http://www.lensrentals.com/news/2010.09.20/lens-repair-data-40





Usability wide open:The Canon 50mm f/1.4 is definitely out of the contention for this category -- it has really bad halation wide open. It's OK on low contrast subjects, but really falls apart on scenes with a little bit more dynamic range. The other two are pretty nice wide open.



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Bokeh:f/1.2 hands down. There is no contest. The bokeh is so much more beautiful that it's not even funny. The Canon 50mm f/1.4 bokeh isn't absolutely hideous, just garden-variety ugly. I'm not sure about the Sigma.


Overall, my advice is to get what you can afford. For most people, I think the Sigma is the best value.