Quote>I'd love a bit more detail on your observations on the 16 vs 17-35. Not cheap investments !<Quote<<<
Here is the way I see it: At f/8 and 20mm I doubt you could tell the difference in image quality.
If you need VR and consistently shoot at f/4 or higher the 16-35 would probably be better for you. If, like me, you do not often use VR and shoot wide open f/2.8 most of the time the 17-35 is much better.