Two totally different solutions here. They both can give you the same results, with the exception that CAD-Duct will be able to go right from the design file into manufacturing. This is known as "design to fabrication".
Both utilize 3D. Both have a catalog of parts. Both run "on top" of AutoCAD as the core. But unless your company manufacture your own duct in-house, and unless all other trades give you a complete 3D file for real world interference coordination, CAD-Duct is not worth the costs or the time for creating a set of mechanical construction documents.