Boat cover vs shed

The problem is my current canvas cover does not do a very good job at keeping water out of the boat when it rains. It is about 5 years old so I expect that. My current shed is only about 7' tall so I can only back about half the boat under it. Hence the bow usually gets pretty wet when it rains even with the cover on. The two choices I come to are to either get a nice cover (Rankin, Evolution) or buy a taller shed that I can put the whole boat under and keep using my current cover. Note that I only use my cover for storage, not trailing. I'm mainly looking for a way to keep it dry and mold and mildew free. Right now I would be better off leaving the cover off because of the mold and mildew. The shed will cost about $1,000 and the cover as you all know will be around $900 or so with shipping. Help me decide which route to go. Thanks.