I cannot help with the Nikon as I have the Sony mount and it was not an option but have had a Sigma 17-70 for a good few months now and am very happy with it. It focuses down to 20cm so can be used as a poor man's macro as well.
I had other choices but the Sigma was the least expensive and I have had a sigma 50-500 for over a year and am really happy with that too.
john
QUOTE (Gerhard @ Jan 25 2008, 12:54 AM)

A: Sigma 17-70mm F2.8-4.5 or
B: Nikon 18-70mm F3.5-4.5