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Go for the Nikkor AF-D.
Buy the Sigma, you'll be making excuses and end up with a worthless lens when you want to upgrade. With the Nikon, your AF should be just as good, as long as you have a decent body, you'll have a tougher, sharper lens, one that was a professional standard for a decade, and it'll still be worth nearly what you paid for it when you decide to upgrade in the future.
A good general rule (with certain rare exceptions) is to always go for the first-party glass unless you NEED a focal range and CAN'T afford the Nikon/Canon/Sony/whatever stuff. Since the real Nikon is cheaper here, it's definitely the way to go.
