Finally I put some comparisons together. https://sojujo.smugmug.com/Testshots/10514/n-MgKPDd
To keep it simple and easy, I just made screenshots and uploaded the PNG files. I think, it easier that way.
Couple of remarks:
The Nikkor has a yellow and the Sigma a blue label (right bottom side)
Most shots were made wide open and not all with a tripod (the fountain and the dragonfly).
While the grass on the river bank was shot form tripod there was a lot of wind going on. Here I don't want to check sharpness - as both lenses deliver outstanding results - but the fringing which distracts me and is not very easy to compensate in post, not to say "impossible".
Bokeh. I could not tell the difference just by eyesight (as long as no front light is involved). In comparison, the uncorrected vignetting of the Nikkor is obvious, but for a portrait lens no big deal (in my book).
AF accuracy and speed. I hesitate to say something about, as my comparison is lacking any scientifically relevant methods, so it's just an impression. In portraits I got more in focus shots with the Nikkor, I guess - I haven't count a number. But the Nikkor was sometimes really surprisingly far off, especially if not the center point but an outer area was selected to focus on. Nothing about AF but one reason for me to hesitate with investment: Often I reached the MFD and wanted to get closer.
Conclusion? Maybe more decision time. The decision is difficult and easy at the same time. Difficult because I like to work with frontlight - and both lenses suck in a lot of photons, not all of 'em treating my sensor nicely. If you see flare, you will not be able to ignore it, it's usually a fat spot. The fringing is so much better on the Sigma that this is a reason to buy the Nikkor only if I really really need one for supercreamy bokeh and headshots, easy to manouevre and acceptable to carry.
I just wonder why Sigma can make a 135/1.8 in a similar body like Nikkor 105/1.4, but goes beserk on a 105/1.4 design. So that makes the decison easy: Bring me a Sigma in a smaller body and take my money.
Not available? No prob, the 85 and 135 are doing pretty aaaaalright.
P.S. I did see coma on stars in the corner. At both lenses' results. Surprisingly well controlled on Nikkor and not as good as Sigma tries to tell us on their portrait-bucket.