Hello,
Some months ago I wanted to replace my Nikon 24-85mm F/3.5-4.5G ED VR cause I wanted a walk around lens with a longer reach even though it is a very good lens. So I tested the Sigma 24-105mm F/4 Art lens.
This lens was terrible even at F8.0-11.0 the corners off the Sigma where soft at all focal lengths.
So I purchase the Nikon 24-120mm F4.0 G lens and I am very pleased with this lens.
Also the Nikon 24-120mm lens is 215 grams lighter and it takes a 77mm filter compared to 82mm on the Sigma 24-105mm lens.
Cheers
Simon