The most obvious difference is that the Argosy motorhomes are painted. Underneath the paint, the Argosy units have galvanized steel front and rear upper caps.
All of them are built and Chevy P-30 truck chassis and almost all of them have Chevy 454 engines. There are a few Argosy 20's that have Chevy 350 engines. There are some early Airstream units with Isuzu diesel engines.
The trim level and options on the Airstream units is higher.
None of the Argosy units will have leveling jacks. Many of the Airstream units do have leveling jacks.
The Argosy units and the early Airstream units do share the same exact floor plans.
I have many examples of both on my website.