Carl, the build on the 17-50 is fine, very solid. It's also a nice size and weight.
On the downside, some Sigma lenses have had problems with using the outer autofocus points on Nikon cameras. This is likely due to the lack of Nikon cooperation with third-party manufacturers regarding the intricacies of the autofocus system. I haven't noticed a clear problem, but then I haven't pixel peeped on most of the photos I've taken with this lens, which have tended to be more on the casual side and intended for sharing after being downsized.
Everything is on hold now due to the D7200 refurb being DOA. The 7200/18-140 might be worth looking at since I have a 12-24 f/4.
