Canon 50 f/1.4, Sigma 50 f/1.4, Canon 85 f/1.8

Yeah I know, another one of those. Hey, at least it's not another "which 70-200 whould I get?"

I'm still weighing my options for an upgrade to my 50 f/1.8 on my 5D2. I know it's tack sharp when used properly, but I don't do that. Mostly I use it in very low-light situations (such as indoors at night), and occasionally when I just want to travel light. So while it's great stopped down, I really need something I can use at 1.8. Unfurtunately, on the 5D2 using the center focus point, when it gets dark enough it simply fails to focus. Even when it does achieve a lock, it turns out that it misses focus by quite a bit. I tried using FoCal on it to see if it needed microadjustment, and it couldn't even complete the tests due to inaccurate readings (whereas my two other lenses worked fine). I'm thinking the AF isn't that lens' strong suit.

So I was fairly set on the 85 f/1.8 but the 1.4s aren't all that much more expensive. Now I know what the reviews say, but I'd like to tap into the collective wisdom here. Assuming I get a good copy, which of the three is going to have the best low-light performance? Not just aperture, but IQ wide open an AF performance as well. I can work with either focal length so it's mostly a matter of cost and low-light AF capabilities.

Thanks!

IQ wide open = sigma
AF in low light not sure. But the sigma has ring USM versus the canon's micro USM.

I find that they (both 50 1.4's) work well for me in low light, with an ever so slight nod to the EF. Just remember that the DOF is so small at 1.4 and if you use the center point and recompose, there is a chance you could be off as much as a few CM, which means the difference between eyes and the tip of the nose or ears!
I don't think you would be disappointed with either, I just like the look and slightly wider FOV that the Sigma offers and smoother background / film like quality. (I also have the EF50 1.4 & 50L, and like them for certain situations)

I have the two Canons.

Since you have a FF camera I am recommending the 85mm. The 85mm is simply a much higher quality lens in every way from build to AF motor and it focuses more consistently, not to mention it sounds much better. It should fringe more than the 50mm but for me the 50mm shows more fringing. It depends on what you photograph, the 50mm is probably pretty good for people but objects with sharp contrast lines (such as black and white writing) make it look really horrible. On the other hand f/1.4 is a solid 60% more light coming in and for "full room" pictures 85mm is too narrow.

There also is the aspect that you can get a 50mm f/1.4 m42 lens such as a Super Takumar, so you can have both focal lengths in low light cheaply.

Canon 85 /1.8 has a very fast AF, and pretty accurate too. On full frame it's about the equivalent of 50mm on a crop; if this FL works for you, I highly recommend it. I love mine.