Rubber and PU timing belts look similar but solve different problems. Rubber is forgiving, cheap, and absorbs shock; PU is precise, clean, and chemically resistant. Most industrial lines use both.
Rubber belts are typically chloroprene (neoprene) with fiberglass cord. They handle oil mist, ambient heat up to +110 °C, and the occasional shock load that would crack a PU belt. PU belts are thermoplastic polyurethane with steel or aramid cord — they are stiffer, run at tighter tolerances, and resist water, chemicals, and abrasion far better than rubber.
For most timing-belt applications, the right first move is to match the material to the environment: rubber for dirty, hot, shock-prone industrial drives; PU for clean, precise, or chemically aggressive drives.