I actually owned the Sigma 1.4 HSM, Nikon 1.8G and now the Nikon 1.4G versions. I can attest that they are two very different lenses.
I loved the Sigma because it was beautiful wide open, soft, but sharp in the center. Gave a really gorgeous look to portraits and it was crisp in nighttime use. Though it is a specialised lens in my opinion, it seemed only very good in the widest apertures from f/1.4 to f/4.5 where it was sharpest. It's a large, heavy, big honkin' piece of glass and it weighs about 1.1lbs.
I love the Nikon because while it's not the best performer wide open, it's better in sharpness and clarity stopped down beyond f/2. It is very good wide open, but only marginally so compared to it's main competition. So for everyday use, I believe it to be a better choice for an all-rounder. Again, this lens is best at f/4 - f/5.6, but it's also not as creamy as the Sigma wide open. But, the all-round-ness, weight (half that of the Siggy), and size of it won me over in the end.
HERE is where it gets very interesting. Compare both to the Nikkor 50mm 1.8G, and the 1.8G wins in overall sharpness, less distortion, and it seemed a little more clear to me. In the end, I opted for that 2/3 extra stop, but at a price. If you don't absolutely need that minimal gain, you'd be best served with the 1.8G.