I vote for a good heavy duty tarp. my buddy used the the really nice, expensive car covers for his MG before he moved into a house with a garage, every year, he had to replace them as removing and putting it on would wear holes in it. even last fall, while his car was outside while the house was being remodeled, the nice new cover leaked, causing the car to leak(what do you expect, its a MG).
now the entire off season for the last 5 years, i lend my 20' x 25" hardware store tarp to my neighbor to cover his boat. bone dry underneath year after year, even when it had several feet of snow sitting on top of it.