-Their ability to deal with flare/CA
-Critical corner/large aperture sharpensss
Sigma is a company in Japan.
Nikon is a company in Japan.
Nikon's website is easier to navigate. Sigma's Japanese support people are really nice. That's about all the comparisons I will make between the two.
Both have great cameras, both have crap cameras, both have great lenses, both have crap lenses.
Now ask me what I think about General Motors vs Ford , or if you want a real answer ask about a specific product you are thinking of buying, you'll likely get a much more meaningful response.
actually with a 2.8 you get 2.8 at all the focal lengths, that's why they are big and heavy compared to the variable aperture lenses.
i have the nikon 70-200 (the old one) and i'm happy with it, but many think it is the worst lens nikon ever built. the main complaint with that lens is that it has ALOT of vignetting. and it does
the new version has solved this problem, so make sure you get the new one if you go with the nikon
i've also got the sigma 300-800mm this is without a doubt a pro lens, it has a pro build quality and feel about it, however i feel the focus is a tad bit slow compared to a pro nikon lens of the same level. i've got the nikon 600mm and that one is super fast focusing--but that might have to do wiht it being a prime lens and the sigma is a zoom.. anyone know about that observation on speed?
just do your research online about the specific lens you want to buy and don't be ashamed buying a third party lens thinking it's just amaterish cause it's not.