Hoover Agitator versus Hoover Activator
There was one machine that could be fitted with either: the Hoover Junior U1104.
Very late models, circa 1990, had the plastic activator instead of the more traditional metal agitator.
Theoretically, the activator should be better, because the brushing power had effectively been doubled (twin helix of bristles) and the beating action doubled too, as opposed to the agitator which was half beating and half sweeping at any one time.
The activator also lifted the pile more effectively and combed it too.
There was another agitator that we never saw much of, but was of American origin: the Quadraflex agitator. Basically, the beater bar rods were situated immediately behind the brush strips. The idea was that, as the brush met the carpet pile, the bristles curled around the beater bar which was now where the brush was a moment ago. As the brush vacated the carpet, the now flexed bristles imparted a kinetic flicking action. Apparently this agitator was fitted to the Mk I Hoover Sensotronic "Electro Kinetic" power head. Supposedly a very effective action.