That is sort of what it comes down to isn't it?
Personally I could never see myself using a fisheye that much. It is a unique lens. And the Sigma is 1/2 the price of the Nikon. So if you are just going to use the lens once-in-a-while, it would seem the Sigma is a good way to go.
But if you are one of these people who use a fisheye somewhat regularly, I say save up and get the Nikon. There is no doubt that the Nikon is a better lens. And if this is going to be an important lens to you, then get the Nikon. Otherwise get the Sigma.