Iv'e been using the fleecy medium priced (about £160) sort of breathable for a few years. Some condensation in winter on outer paint, though mines parked on the kerb in quite a windy street, so gets a lot of air underneath and always seems dry there.
I leave the windows a bit open and have never had problems with damp/mold inside.
Mine have never suffered noticeable wear around the back fins, though I imagine the thinner,more plastic ones would.
I prefer using a cover, as there is quite a few nooks and crannies on these cars, where water will sit.