I have both of these lenses. In my opinion, they complement each other more than replicate each other.
The Nikkor is a bit faster and I have tested it to be sharper, even at 300mm, than the Sigma at 300mm at the same f-stop. It is also much more portable and ideal for use inside terminals or when on a non-spotting vacation that you just happen to find time to spot during, haha. I also use it when I know I need to get the best possible shot of a once-in-a-lifetime opportunity like the Space Shuttle on top of the SCA last April.
The Sigma...is the best bargain you're gonna get at consumer prices. You got the money for the pro glass, spend it. But personally I know I would feel constrained by the narrow focal range of the 200-400 as 200 is too long for a short end and I like being able to get to 500mm with the same lens. It's a big lens with a cheapo finish and a cheapo lens hood, but the glass does pretty well if you stop it down. The lens acts up sometimes leading to focus locks, and it sometimes doesn't play nice with my D90 and gives an error message. I know this is from the less-than-perfect seal at the lens mount. I only got it because I got tired of waiting for Nikon(are you listening, Nikon- I gave someone else money I would have gladly given to you had you done what everyone is asking for and refreshed the 80-400 but you played games with not one but TWO new 85mm's, so I went with your competitor) to come out with something that was longer than the 70-300 but didn't cost a fortune and had AF
That said, I have taken many fine shots with the Sigma and feel that it plays a valuable role in my range of lenses. I use it when I know I will be at spots where 150-500mm would be perfect(airshows, my back deck, some spots at JFK
, most a/c at PHL
). To those who say it eats a lot of light, well, shoot when it's sunny or raise your ISO since most cameras now have pretty decent ISO performance up to 1000 or so.
Hope that helps