One difference I seem to remember from my days working as a bike mechanic in the '70s was that all the Campy rear derailleurs used solid bushing pins for the four pivot points whereas Suntour used folded-over sheet steel for most of the models, with the result that the Suntour derailleurs got sloppy over time.
The top-of-the-line Suntour derailleurs probably had solid pivot pins, though, since they were such exact copies of the Campy parts, at least for a while in the early days, many of the Suntour and Campy small parts were interchangeable.