So, I just got a Sigma 50 f/1.4, and after owning the 35L and 85L for a while, and the normal focal length being my most often used of ANY lens, I thought 'the Sigma's good, but what if I'm just stalling the inevitable.' So, I just went out and bought a 50L, deciding I'd return the one that didn't impress as much.
Well, after a day of shooting around with the 50L, and a day yesterday with the Sigma 50, and a bunch shooting them together (mostly of each other)....the Sigma is staying. Definitely.
First, Bokeh: Identical. Honestly, both are such incredible bokeh monsters, and there's really not much to separate them. If I absolutely had to pick, the Sigma would come out on top, as the specular highlights are just a SMIDGE softer...but both are really among the best of any lens I've ever tested.
Sharpness: Again, relatively close...when the Canon was in focus, it might be a hair sharper at f/1.4 vs f/1.4, but that's really splitting hairs. (and the Sigma might be a smidge sharper at f/1.4 vs f/1.2)
Focusing: Well, this is the real reason the Canon is going back. The 50L I got has got a serious backfocus problem up close (and it switches to front focus on distant objects, interestingly. Consequently, it's pretty good at moderate distances.) Regardless, it missed focus a LOT. My Sigma has been quite accurate. Under controlled conditions, it's been very accurate. I had a few front focused shots when shooting my daughter, but given the accuracy of many other shots of her, and the accuracy under controlled conditions, I think my Sigma focuses just fine. I'm holding this against THIS 50L, but I'm not holding it against the 50L in general, as I took a few shots with manual focus to see the real performance of the lens.
Contrast: Tie...both are quite good, even at wide apertures.
Highlight blooming wide open: Both suffer quite a bit from this at f/1.4 or f/1.2 on very high contrast subjects, causing blooming around whites. Common with very wide aperture lenses, but a little disappointing considering how good the 35L is in this department.
Longitudinal CA: Both have it, but the Sigma's is much better controled. The Canon has longitudinal CA out the wazoo at wide apertures..of course, only really noticeable on very high contrast subjects.
So...overall, both are very good lenses, but I honestly would say the Sigma (so far) appears to be the superior optic. I'm returning the 50L tomorrow, and I'll continue to shoot my Sigma. If it ever begins to disappoint me, I might try a second 50L to see if the focusing issue was a fluke, but I honestly don't think I need to try another.
Here are two shots of each lens, both taken with the other lens at f/1.4. I won't show crops, though the Sigma is sharper here, as the shot of the Sigma with the 50L is slightly missing focus.
Sigma 50 @ f/1.4:
Canon 50 f/1.2L @ f/1.4: