Can you see the difference between these nikon vs sigma lenses? Yes. Is it worth $11,000USD more? Mmmmm sure, because it shoots at f/2.8 and the other at f/6
The top Nikon glass is made for sports photographers who need the aperture and reliability. They keep working and won’t complaint. Not to say the Sigma doesn’t, but its not made to withstand the abuse like the Nikons. If it breaks, you can buy 10 more. But you will never be able to shoot at f/4 or f/2.8?
and here we have the Nikon 600mm f/4 FL VR
AF-S G NIKKOR