Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Nikon AF-S 120-300mm F2.8E FL ED SR VR is the longer of the two lenses at 304mm. The Sigma 120-300mm F2.8 EX DG HSM with a length of 271mm is 33mm shorter. Besides being longer, the Nikon AF-S 120-300mm F2.8E FL ED SR VR also has a larger diameter of 128mm compared to Sigma 120-300mm F2.8 EX DG HSM's 113mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Sigma 120-300mm F2.8 EX DG HSM weighs 2600g, 25% (650g) lighter than the Nikon AF-S 120-300mm F2.8E FL ED SR VR's weight of 3250g.
Below you can find the dimension and weight table of the 'Telephoto Zoom Lenses in Nikon F (FX) Mount for size comparison purposes.
Telephoto Zoom Lenses in Nikon F (FX) Mount for Size Comparison
Lens Mounts
Both Nikon AF-S 120-300mm F2.8E FL ED SR VR and Sigma 120-300mm F2.8 EX DG HSM has the same Nikon F (FX) lens mount. Some of the latest released cameras that are compatible with the Nikon F (FX) lenses are Nikon D6, Nikon D780 and Nikon D3500.
Sigma 120-300mm F2.8 EX DG HSM is also available in Canon EF and Sigma SA mounts.
Focal Range
Both lenses have the same focal range of 120-300mm and the fastest aperture of F2.8.