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I owned the Nikon 35/1.4G, currently own the Sigma 35/1.4, and tried the Nikon 28/1.8G.
I had poor focus accuracy on the Nikon 35; the Sigma is much, much better. The focus on the Nikon was so inconsistent compared to the Sigma, that (ironically) reminded me of Sigma's older offerings. The Sigma 35 is definitely a gem. Not only is focus better, image quality is superb at f/1.4. The micro-contrast from the Sigma reminds me of that from a Zeiss 35/2.
Comparing the Sigma 35 to the Nikon 28, they are two different focal lengths, so it comes down to preference. Shooting a Voigtlander 28/2 on Leica M-mount, I personally like the 28mm focal length.
Now looking at price, the Nikon 28 and Sigma 35 are similar. I went for the Sigma simply because I feel it's the better lens in terms of image quality. I also need f/1.4.
Being in Canada, the Sigma also provides a 10-year warranty with Gentec International. This definitely adds value to the Sigma for me. I no longer worry about compatibility with future bodies, front/back focus, and any mechanical issues with the lens. It's all covered.
Hope this helps.