Had both robustness comes at a premium with the H&B expedition panniers, they weigh considerably more. The Touratechs are lighter but that also means more bendable. On impact they are likely to mis-shape rather than break, and a hammer will straighten them (to a degree).
Don't know the weight of Verns and again it depends on the size as to how heavy they might be but I believe them to be mightily robust
The mounting frames and mounting system of the Touratech are the stronger of the two between Hepco & Becker as I see it.
Don't know about Vern's though, never seen them, but the quality of the photographs and from what people have been saying they must be worth an extremely close look.
I've an Mpeg demonstrating just how air tight Touratech Zega panniers are. This Mpegs shows just how difficult an increase in air pressure makes it to remove the lid. The clip was taken at sea-level after the boxes were last opened at 1500m and 90min earllier.
It's 5mb in size. Let me know if you want to see it.