At such a price, this Sigma is highly over-estimated, IMHO. I bought my Tamron 90mm f/2.8 for <400$ (rather close to 350$, if my memory serves me well). Keep that Nikon - the difference in price is reflected it difference in quality. Also, of all third party producers, Sigma's lenses are best known to be suffering from incompatibility with some "brand" camera models (such and such lens doesn't work with such and such Nikon's or Canon's model, while perfectly working with other models)...
edit: what "FredKingston" just posted: according to many reviewers, Tokina's model might be the best in it's class, out there, in terms of quality vs price. At the time I was about to get me a macro lens, it was not available in Serbia, so I purchased Tamron.