Corten (A606 - thin sheet) weathering steel has published properties slightly better than CRS. It's a common choice in steel boatbuilding. In comparison over the years, I've found it to have, on average, alot better deformation properties than commercial quality CRS. For example, it generally takes a 1 to 1.5 times thickness inside radius to form.
Also, being in my experience a harder material, there's the abrasion upside. Welding? It's great, and heat propogation is very low... a big plus in the boat building area, alot less likely to get the "starved horse" effect. Lastly, corrosion resistance.
Just a heads up. The "U" shaped piece shown in one of the pics you found, is a tube gusset, not a frame rail piece. On the first build, I had the rear tube work mounted to the frame rail doubler, and used that piece to make the joint on each side.
--ron