The OP states the roof was leaking around an antenna, not through the whole roof. A complete new roof is expensive and,IMO, the older sheet aluminum roofs are superior to EPDM covered roofs.
The essence I'm getting from the posts by the OP is that he hopes to get the cabin temperatures lowered in the summer and to replace the poor insulation job in the back wall where he has to open it up anyway. Any insulation material is going to be towards the hopes of the OP. Spray will be the best R-value and the most expense. Extruded polystyrene will be close to spray in R-value, less cost but more work. Fiberglass or Roxul will be the least expensive but the lowest R-value.
One thing to consider is the dimensions of the spaces involved. There is not that much difference in a 1 1/2" wall if filled with the best or worst insulation. The ceiling cavity with the larger space and exposure to direct sunlight will benefit the most from the best insulation.
