Water proof cover is better in resisting the rain waters harmful effect on the colourful paints. But the diffusion of water particles from the cover to metal surface is inevitable. So once the rain is stopped it is better to remove the cover and let the vehicle dry.
Air breathable cover has an added advantage over the water proof cover in resisting the moisture enters into the metal surface. The open atmospheric air has some percentage of moisture in it. In science terms it is called humidity. The humid in air starts to diffuse into the metal surface even the rate is slow. Still it corrode the surface because the car surface is going to be in contact over a long period to time which simultaneously accelerates the corrosion once it is started to corrode. The air breathable cover will assist the air to flow easily and wipe out the moisture. Also the hot air starts to condense under the car cover. This has a similar effect just like humidity in air. The air breathe cover does not support the condensation. Some artificial fibre covers are available to protect the car in every aspect. Whereas the water proof cover does not prevent the condensation. So it is obvious the air breathable cover is superior to the water proof cover.
When it comes into the aspect of weight, the water proof cover are so heavy that implies a heavy load on the side mirrors and damage it if handled roughly. The fibre air breathable covers are less in weight. So the damage is so minimal and negligible effect. The modern cars have a specially designed car cover that fits for the particular car mode. It comes as a freebie.
What a Car Cover should have?
The car cover should serve the purpose of protecting the metal surface for a longer life. There are different car cover like outdoor and indoor covers. They are designed for different applications. The following aspect is expected from a branded or non-branded car cover.
The cost of the car cover is expected to be minimum at the customer end.
The car has to be protected from UV sun lights, snow and Rain. The cover should not get damaged in all different environment.
The cover should be very soft so that it will not scratch the metal surface. Also should be air breathable.