If you are in a rainy country, then an open-air portable canopy is the way to go I feel. A car cover inside of a dry closed building works fine, but if the car is sitting outside in a humid atmoshpere, then moisture, dampness and mildew will collect under the car cover and eventually cause damage to your car in the way of surface rust, corrosion, upholstery rot and so forth. Once inside of the car cover, the moisture cannot really escape that easily. Also, don't forget about snow and ice sitting on top of the car too. With an open air type of canopy, then the car can breathe, and any moisture that does collect can then dissipate as well.