If your building is located in an area that receives a significant amount of hail storms, your best option will probably be EPDM. It is made of synthetic rubber. This causes the hail to bounce off with almost no damage to the roof surface or system. TPO is made of thermoplastic. When it is struck by hail, the hailstones produce gashes in concentric circles that remain in place. In the future, this could compromise the membrane of the roofing system.