I don't own a Nikon, but I can definitely vouch for the Canon mount of the Tamron 17-50
f2.8. Very sharp even wide open, and a wonderful range.
If you already have a Nikon kit lens (the 18-55 I assume), you're not going to very much better off with the Nikon 18-70, because the maximum aperture is still the same. Believe me, f2.8 makes all the difference in the world, in terms of being able to shoot indoors and being able to get a nice bokeh in the background, making the subjects in your images "pop". And being able to shoot with natural light rather than using a flash makes a very big difference in the quality of an image.
You will definitely want to get a wide zoom starting at 17mm, because the creative possibilities of a wider field of view are much greater than that of a fixed 50mm lens.
For what you will be getting, $600+ is very good value.