I went out and bought the Nikon 1.4G after reading this thread, many other sites and conducting my own shoot tests.
The Sigma is one phenomenal beast of a lens, a 77mm dia and 1/2Kg of weight, wow! The sheer amount of light it captures is amazing. The bokeh is indeed better on the Sigma. The build quality is also at par with Nikon if not better. My dealer (whom I trust) told me that this lens is a big hit with those shooting 1080 videos on Nikon bodies.
I chose the Nikon 1.4G because in my test shots it did provide better edge to edge sharpness, something I need when putting the subject away from the centre (happens often), also because I can't risk getting a bad apple Sigma lens (there are reports of this being a persistent problem) and finally cause I need a lighter 50mm, already have lotta heavy lenses. The bokehn is also pretty world class on it, though not as buttery as on the Sigma.
Right now I'm on D2x, once I move to an FF body (if the powers that be at Nikon decide to finally come out with a D800) I will need even better edge to edge sharpness. But if ever I get tired of the Nikon 1.4G or want to try a more "arty" product the Sigma 1.4 it'll be.