What is the anticipated lifespan of your model?
If it is a year or less cover it with a film, any will do and they all will begin to sag or wrinkle within the year. Monokote also becomes fragile and brittle after a few years requiring constant patching. However, the labor involved with applying the film and maintaining it will be manageable IF the plane doesn’t last long.
If you anticipate the model to last longer than a year, the labor expended in maintaining the film will soon exceed that of applying a quality painted fabric finish. Fabric and paint is therefore the best choice.
I have used Sig Koverall on planes powered by .10 glow engines and I can assure you that when properly applied it can be as light as film. However, you must accept a finish that looks like fabric, if you desire a film like finish then fabric is the wrong choice and will indeed be too heavy.
Another choice is Polyspan which works very well on planes smaller than .40 glow size. It is very strong and very light but has some unique application properties.
The choice comes down to how much effort you want to devote to the project and when to apply that effort. A fabric/painted finish is maintenance free once completed and a film finish requires constant maintenance after completion. The fabric finish will require more labor to apply than film.