Nikkorr Vs Sigma

As long as they have aperture ring and cover full frame, they will work. (The ones that do NOT cover full frame have the DC designation (same as DX on Nikkors))

So the 20-40 will work, but I have no idea about the optical quality...

The best lens in that focal range is the Nikon 17-35/2.8. If you want to spend less, the Nikon 20-35/2.8 is a good buy on the used market.
 
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