Monofilm is a clear film. X ply is crossed fibres laminated between 2 sheets of thin film.
The benefit of x-ply is that if punctured the tear normally only travels a few inches, whereas the monofim has no tear resistance and the panel will normally tear in 2.
The benefit of x-ply is that it tends to be more durable, so it tends to be used for wave/crossover sails
The benefit of mono film is that it has a small performance advantage, so it tends to be used for race/speed sails
In the pictures below the translucent panel in the Vibe is film and the translucent panel in the force is x-ply.
Ezzy sails probably have the reputation as the most durable.