From a previous post:
"the sigma 70-300/4-5.6 APO Macro and 75-300/4-5.6 APO Macro are both
lenses that need to be stopped down a couple of stops from wide open
to get edge-to-edge sharpness"
"imho these lenses are bordering on useless handheld if you want the
most critical image quality. my impression is that the sigma's are better than the non-L canon 100-300 lenses"
"both lenses have the "legendary" sigma mechanical build quality, ie
they are not very well made mechanically"

I am mystified by something I have often seen: a lot of money spent
on a top Nikon body, like an N90 or F4, and (relatively) little
spent on marginal (relatively) cheapo glass. The glass determines
the image quality that is placed on the film (merely) held by the
camera body. Any old Nikon body will do, but not just any lens for
good quality images. In reference to the above, either the 100-300mm
f5.6 Nikkor MF or the 75-300mm f4.5-5.6 Nikkor AF can be used
wide-open (good center-to-corner, not just to the edges), and are
well-made. In other words, they are good, useful lenses (which will
also hold their value on resale [and most of us do eventually sell
our lenses...]). Cheaper off-brand lenses are no bargain in the
short or long run for a Nikon user because of inferior image quality
and lower resale value. Hope this helps.