r-30's worth the extra
My shop is a metal building an I have r-30 in the walls and ceiling. The inside walls were done with siding metal also. There is an extra benefit of the r-30, it really makes the place quiet to work in.
I have floor heat which is the best thin I have ever done as it costs little and keeps the place warm and equipment also. Summer I keep the door closed and it place keeps cool. I was going to use AC but didn't need it or the expense, I just use a dehumidifier.
The thing I would do different is insulate next to the roof and then the attic storage would stay warm in the winter, I put the insulation between the ceiling and the attic. I use an exhaust fan in the attic that comes on at 85 degrees, and it runs in the afternoons.