I loved the IQ from the sigma 120-300 but it was a PIA to hold. The internal zoom did make it perfect on a monopod for sports....I think my last one, the OS version, was about 3500 new so 10k is a joke.....I doubt anyone would see 3x better images or 3x better anything
At $10K, Nikon is only helping Sigma sell lenses. I had the version before the Sport current model and aside from being a heavy beast, it was a great lens, especially for indoor or night sports. As Randy said, I highly doubt the Nikon model will be 3X better.
I read about the Nikon 120-300. My impression is that the glass is great. I do not care for the screw-on filter or the size of the filter. I am sure the cost of the screw-on filters for this lens will be high. Further, from what I can see if you use a CP filter, you can only adjust it with the lens shade OFF, like the 500 PF. I find the 180-400 with the drop-in rear filter to be much better as you have the ability to adjust a CP with the proper rear filter holder. I believe the adjustable rear holder and filter are the same for all the super-tele Nikon glass. Just my opinion.
Really disappointed with the price when I saw it.
I thought about buying the Sigma 120-300 f/2.8 Sport after working with my 70-200E and 500PF w/out something to fill the gap. I am a wildlife shooter and do not have issues with the 300mm limit, as I shoot fairly approachable wildlife, use both DX and FX bodies in my workflow, often shoot from a blind, and rarely shoot small birds. After playing the the Sport lens, I decided that the balance shifts from 120-300 were a real issue and the weight savings over the 200-400VR were negligible. In the end, I found another (#3 for me) 200-400VR. This lens was $1700 (can you believe it!) and looked to be unused. I've had it for about 3 months now, and it is the sharpest of the three I have owned thus far. I think that I am pretty set w/ my 70-200E ($2100) + 200-400 f/4 ($1700) + 500PE ($3600) = $7400 ...all for less than either the new 120-300 or 180-400 Nikon lenses... would love the latest and greatest glass, but I doubt that any of those new lenses would translate into "real" noticeable differences in my images.