The short version is that in my experience, at night, wide open or at f/2, the Sigma 50mm f/1.4 HSM has less coma but is not quite as sharp in the center vs the Nikkor 50mm f/1.2 AIS. At the edge of the frame the Sigma has a slight lead in sharpness. Read on for more if you like detail.

There is a very detailed review of many 50mm lenses here, but a fast 50mm lens is not for use in bright daylight.

In the process of capturing the above image in 2011, the hunt began for an ideal 50mm for shooting stars. The above image looks good online, but in full sized view the image pretty soft, and the exposure was a bit too long at fifteen seconds and the stars leave small trails. Plus there is some noise because the original was too dark and had to be pushed a full stop in post-processing. In general, 50mm lenses are cheap to make, and have been in production longer than SLR cameras. So there are plenty of options out there. The obvious choice would be one designed specifically for night, Nikon's 58mm f/1.2 Noct-Nikkor, except for its $4,000 price tag. I don't take that many night sky shots, or have that kind of disposable income. Just recently I picked up the Nikkor 50mm f/1.2 AIS and Sigma 50mm f/1.4 HSM and got a chance to shoot them an a night sky.

A quick note on each lens. The Nikkor 50mm f/1.2 AIS is a manual focus lens that has been in production for over thirty years and retails at $700.  It's beautifully built to the highest classic Nikkor quality and has a standard of the era 52mm filter size. It holds a reputation as Nikon's sharpest 50mm lens at f/2 and is currently their fastest lens in production. Like all fast lenses of its age, images have a "glow" wide open with notable loss of contrast. Although lighter than the Sigma at 13oz, the all metal build makes it feel like it could last forever. I picked mine up used in beautiful condition for $450.

The Sigma 50mm f/1.4 EX DG HSM is a new lens on the market retailing at $450. Build quality is standard for a modern high end Sigma, it's nice but doesn't offer anywhere near the tactile and visual appeal of the old Nikkor. It's also heavier at 18oz and uses a 77mm front filter. The large filter size might be an issue for people new to photography, but it is the standard for all high end Nikkor f/2.8 zooms, which in reality makes filter sharing more realistic than the Nikkor for most modern photographers. As you can guess the 77mm diameter makes the lens big, and this lens is well suited for a FX camera and will feel big on DX models. Sigma was obviously shooting for the higher end here when they produced this lens because at the time of release it was more expensive than any 50mm f/1.4 Nikkor, though now it's matched in price by the latest AF-S G version. It's also worth noting the Sigma is more of a 45mm than a true 50mm focal length.
   
Researching these two lenses I came across some interesting opinions. F-stops are just a mathematical equation of what the light transmission of a lens should be. T-stops are the actual measured transmission values. I saw many claims that the Sigma f/1.4 actually allowed in more light at f/1.4 than the Nikkor at f/1.2. According to some, the digital sensor does not react to light coming in at an angle as well as film, making fast lenses less fast in reality on a digital sensor. Also according to some, because of this problem, a digital camera will actually increase the ISO behind the scenes so that the old fast lenses expose and meter like they should. So that raises the question of how do we test the actual t-stop of a lens without complicated equipment? Simple enough in my case, try both lenses on the Sony NEX-7 because it will shoot with no idea what lens is attached and therefore has no reason to change the ISO. In tests on the NEX-7, the Nikkor 50mm f/1.2 shows an imperceptible difference between f/1.2 and f/1.4. At either aperture it lets in slightly more light at than the Sigma f/1.4 but has notably lower contrast. On the other hand, mounted on the D700 at night the Sigma f/1.4 produced brighter images, perhaps confirming the ISO boost behind the scenes theory, or just a different sensor to lens response. I'd also guess that with the 77mm front filter is has a lot less vignetting.