15 X 20 is a large shed, I wonder if you require a building permit and, if so, is it required to be built in compliance with a building code. Assuming you do not require a building permit and do not have to comply with a building code, then a course of concrete block would be the way to go, your framing and sheathing (and wood siding) will last much longer. If you need to comply with a building permit the 4" slab on grade will not comply with most codes in most climates.