better in sharpness and accurate focus
The lens is only one factor in getting you sharpness. For the sake of simplicity, I will assume all other things equal (and on an acceptable level).
The Nikkor is slightly sharper wide-open if we approach if from a corner-to-corner aspect. The Sigma is better if we approach it from a center-only aspect.
By f/2.8 the lenses are more or less equal.
Both are the same regarding focus (=it depends on so many things that it becomes meaningless to judge this only by lens alone).
But honestly, you're approaching this from an entirely erroneous (and un-photographic) perspective. Both of these are great lenses, sharp, contrasty, capable of delivering stunning images in the hands of someone who knows what they're doing. I've made most of my photography money using an ancient AF-D 50mm f/1.8
Your choice is simple:
- Do you need the f/1.4? If yes, get the Sigma
- Does the f/1.8 suffice? If yes, get the Nikon; spend the rest of the money at your local pub.
