Slight correction: the Nikkor is one third stop faster, and only at the long end. (f/5.6 vs f/6.3. A full stop faster would be f/6.3 vs f/4.5.)My wife and I have both the Nikkor 18-200VR and the Sigma 18-200OS. Both are sharp and are great walkaround lenses. I find the build better on the Sigma but the Nikkor is one stop faster.