Both on my D700, I've used the Sigma, used the 50G, bought the Nikon.
I couldn't rely on the Sigma's AF, and the lens
sort of defeated the purpose of a 50mm for me because it was so big. It's a beautiful lens though when the stars and planets align and it looks sweet on a D700.
Reasons for the Sigma:
It takes 77mm filters. so if you have big glass already, its great for your polarizer and ND filter.
It's got slightly smoother Bokeh. Slightly.
It's slightly brighter then f/1.4. Again, Slightly.
It's slightly wider then 50mm. But only if you look for it.
Short focus throw. AF is quick, for sure.
It doesn't vignette so much. But on FX, the corners are crap regardless of aperture. DPReview and SLR Gear both confirm this.
The overall look. It's smooth, and pretty sharp wide open near the center. Where you're most likely going to use a fast 50mm.
Reasons for the Nikon 50G:
It's small. That's one of the reasons for a 50mm, it's discreet.
It's sealed. Ass-gasket and all, put a UV filter and the hood on it, and you can shoot in the rain!
Handling. Both the 1.4D and 1.8 have rotating focus rings, making camera positioning a pain in the rear because you've got to watch your fingers.
AF-S (even though it's slower to rack in and out). It is so dead on, it's uncanny. And what good is f/1.4 if it's going to be out of focus?
Long focus throw. Manual focus is cake on this.
The overall look. I love the bokeh wide open, and the vignetting wide open. Not to mention, this thing can cut glass at anything between f/2 and f/11, then past f/2, the Nikon's more uniformly sharp (
dpreview.com - Lens Review - Fullscreen)
Oh yeah, and with a D700, it's like night vision.
f/1.4, ISO6400, 1/45th of a second. The only lighting was a string of shop lamps at 9:30 at night. If I had used the big-ass-holy-grail-motha-fu*ker 24-70, i'd have to shoot at ISO 25,600 to get the same exposure. Yeah right. 25,600 looks like dog sh*t. Basically, when the sun went down, i would have had to put my gear away and I would have a massive midrange stuck in peoples faces in the meantime.
I just wish the 50G had a petal hood like the sigma, that would be choice.