[stoppingdown]

I've just run a quick comparison between the two lenses, with my Sony A6000 and manual adapters. Since the Sigma diaphragm cannot be controlled by the manual adapter, it's at full aperture, which at 300mm is f/6.3. The Nikkor has been manually set at f/5.6. At the center, the Nikkor is slightly sharper; but at the corner the Sigma wins hands down.

 

At 500mm I compared with the Nikkor + the 1.7 teleconverter; I left the diaphragm at f/5.6, even though the resulting aperture is f/9, because I think that the lens performs a bit better stopped down. In this case the Sigma seems a bit better - let's say it's a tie, because there could be manual focusing error; at the corner the Sigma wins again.

 

Very good considering that the Sigma is at full aperture - and the Nikkor is a prime, even though old.

 

The images have been post-processed and sharpened as per my usual settings. Also note that there is some atmospheric blur (the subject is 600mm from my window). I've used the self-timer to avoid shake. I took 3 shots at each setting and then picked the sharpest one.

 

Consider that the Sigma lens is brand new, while the Nikkor saw more than ten years of adventures... I think it is ok from the optical point of view, but I can't exclude that age has slightly decentered it.