I was going to get the Sigma but end up got the Nikon as the Sigma i ordered has focusing problem.
To be honest, the two has very similar real life performance, so you can't go wrong with either one. But if you want to buy the sigma, just make sure you buy it from a reputatable shop that allows you to test/return if it has focusing problem.
Sigma 50mm f/1.4 EX DG HSM is slightly better in term of "bokeh", it has aspherical element where Nikkor 50mm f/1.4G doesn't has...
in comparison for picture quality, there are really quite the same (really good)...
aspherical element help to reduce distortion and improve the sharpness at the image borders...
this distortion is also known as chromatic aberration (CA) or colour fringing...
Sigma len is bigger and heavier than Nikkor.
As silverxu and yellowS15 said, make sure to buy the len from reputable shop so you can change it if it has front/back focusing problem...