Both are really good. I had a Sigma 17-70 and thought it was superb, 70mm is spectacular, less so on the wide end, where I feel the Nikkor has the edge.
At the time I had the Sigma, I got a D60 so replaced it with a Nikkor 18-70 DX for the AF.
The Tamron 17-50 is also work a look, its the best out of the 18-70 DX and Sigma 17-70, although you may find 50mm a bit short.
But all of them are excellent, and you can't go wrong with any. As they are all good, you would do well to consider just picking up the cheapest in this instance.