Considering the potential for mold or mildew to form between the cover and the tarp, I would suggest one or the other. If your area sees snow, assume the worst and make sure whatever you use is well supported. The "ridge pole" plan, running from front to rear, with supports every few feet, will hold a lot of snow, while minimizing potential for a damaged cover/tarp. Last, make sure air can circulate under the cover. If you get a late start on your season, and your cover was a little to weather proof, you can have a nice black mess to clean up when you pull the cover.....