There are some issues insulating a metal sided shop that will be heated, the warm air inside will allow water to vaporize and move through the insulation to the inside of the metal siding and condense there causing moisture (insulation can become wet etc). Moisture that does move through can get trapped in the insulation and along the walls. This is the great advantage of the spray on foam- no moisture can move through and condense on the walls as you have essentially coated the metal. There seem to be two schools of thought when using fiberglass in this situation- one is make sure you have a good vapor barrier on the warm side (inside the shop) of the fiberglass so moisture can't move through and get into the insulation. The flip side of that is that you can't ever have a completely sealed vapor barrier and some moisture will get in, thus some say not to put in a vapor barrier so that moisture that does get in can vaporize and move back and forth freely. In a house that uses fiberglass, the siding and housewrap allow any moisture that moves into the walls to move out through the house wrap and siding, but metal is not permeable in the same way. That being said, I insulated my shop with fiberglass batts and put up a 6 mil vapor barrier sealed with tape- inexpensive and effective and do not have moisture problems that I am aware of. If you can keep the insulation from touching the metal siding so much the better- any moisture that forms on the metal won't soak the insulation. The spray foam has some advantages of keeping wind out, no nesting for critters and reportedly stiffens up the whole building. You also have to think about your climate, high humidity worsens the moisture movement and I don't have that where I live.