The 17-70 is only 2.8 for a few millimetres at the wide end. For portrait lengths you are looking more around f/4 as the maximum aperture.
As I discovered my nifty fifty has a back focus problem I ended up using my Sigma 17-70 at Christmas with my nephew (saying that makes me sound older than I am!) and there was not enough depth of field.
Of third party lenses the Tamron 28-75mm lens is very popular and has a constant 2.8 aperture, as does the Sigma 24-70mm EX. Both are usually about £50 more than the 17-70mm, while Sigma also has a 28-70mm f/2.8 EX for the same price. I do not know anything about Nikkor lenses but if they are anything like Canon's equivalent models in this range they will cost twice as much as the above and unless you need faster and quieter focussing will not justify the extra.
Michael.