If the cars staying in one place for a while and you've got the room, you'd be better off with some form of
Cartent (pretty much just a gazeebo, i've got an old caravan awning that does the job
)
If you go for a cover, make sure you give the car a good protection layer prior to using the cover. That way the wax / sealent will hopefully take most of the rubbing and protect your paint underneath. I've never liked car covers
