I'm doing 1.5" on walls and 2" on ceiling. Closed cell goes beyond R value because it's vapor tight, air tight, radiant heat resistant, and adds rigidity to the building. If you chose to go closed-cell, have them house wrap the exterior before they put up the steel. If you ever get damage to the steel, you can't remove it from the close-cell without putting a gun in your mouth.
My insulation will cost as much as my material for the whole building (not including concrete).