I've not tried the Nikon but it would be some lens if it can top the Sigma 35mm. I'm in the process of switching back to Nikon and the Sigma 35mm was the very first lens on the list. It's also the only one that was an absolute must. All the other stuff, I felt there were two or three options that I could happily opt for. Funnily enough I don't even need a 35mm, it sits awkwardly with the other lenses I want, has no specific purpose and yet sits right at the top of the list! So, hope that conveys my sheer enthusiasm for the lens. It's by some margin my favourite lens that I've ever used, and there have been some good ones!
Being a little more objective about it, downsides to the Sigma are higher price (I assume?), you probably want to get a USB dock to get the best out of it and lastly it's quite a big lens and certainly quite heavy. I assume a lot heavier than the Nikon 1.8 but then it is a 1.4 and feels every bit like a 'pro' lens.
I like the look of the Nikon, if I'd never tried the Sigma, it would be right up my street but the Sigma has a little magic.