> I have no idea if a mid range zoom is sharper than an everything zoom like the 28-200mm.
I doubt it matters much. As long as one or the other are not exceptional - good or bad - it's straightforward to choose. As examples, pretty much any macro lens is exceptionally good. The Nikkor 70-300 G non-VR is exceptionally poor. I had assumed that the 28-200 was in fact the Nikkor 28-200G, which is pretty notably better than the Sigma version.
> I am becoming a snob for metal mounts.
It's actually the small minority of lenses that do NOT have metal mounts. It may surprise you, but I can only remember one instance where someone destroyed a lens with a plastic mount. That's actually equal to the number of people - me, in this case - who destroyed a lens by physically separating it, although my lens did have a metal mount. It's really not much of an issue at all.
> I dare not change a lens in the dusty environment of the shop.
I agree in theory but not in practice. First, it's not as if some dust will destroy the camera. It's not even going to destroy an image. Second, I've changed lots of lenses in worse environments and lived to tell the tale. I doubt, for example, that a fully working roundhouse with live coal-burning steam locomotives and a fully operating machine shop produce less dust than your woodworking shop, yet I changed lenses as photographically necessary on such a trip and I don't think that I had much if any sensor dust issue as a result of it. Third, your D90 has one of the dust-shake sensor vibration systems, so even if you had gotten a little dust in the mirror box, it probably wouldn't have affected your images. (My D3 does not have such a feature.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!