Here's my experience with covers;
Indoors a flannel cover is best, fit isn't as important, but the better it fits the happier I am as it prevents any movement.
A generic cover outside is a waste of time, the only covers I've had success with are the ones that are flannel lined, breathable and a snug (designed) fit. Everything else blows off or leaves marks on the car, it's critical it fits with no room for movement.
The dealer supplied ones have been good, a California Car Cover I purchased for a Mustang to fit on a Mustang was terrible.