I am in the same dilemma. I have a 52 x 42 x 12 Morton Building. I ruled out metal due to the difficulty in mounting shelves or anything else on the wall. The ribs in the metal would keep shelving from being flush with the wall.
I then thought about using OSB, and painting it white. The price is right. I'm just not thrilled with OSB. Drive a screw near the edge, and it self destructs, which could present problems if you want/need to remove panels in the future. I'm looking at plywood at this time. I think it will look better than OSB with a coat of paint and also believe it will hold up better. Yes, it is more expensive than OSB, but I want to do it once and be happy.
I have a friend who just built a smaller Morton building. He is looking at plywood for the first 4 feet, then metal above that.
Good Luck!