Why go to the expense of building a sub floor and then covering it with plastic for moisture control? Just do the thick plastic with moisture control. If you are thinking about putting in the floor so you don't rip the plastic when you store in it you don't have to. They make plastic membranes for this purpose that won't tear.
Pick up the new issue of Fine Home Builder, they have an artical this month that fits your project perfectly about moisture proofing a crawlspace and turning it into storage.