Dex 6 spec technically supercedes the Dex III spec. The main difference is that Dex III has a higher starting viscosity, but Dex 6 is more sheer stable. Both Amsoil products have similar viscosity properties however. I believe you are over thinking this. Neither one will cause a problem. Torque drive is an older product, but still popular in trucks and buses. From the technical specs, it appears to be a better fluid in general. The main difference is that it comes in large containers, and is made to be compatible with heavy machinery transmission fluids.
2005 Obsidian Black Pearl XT w/4eat