Solid mounts are exactly that; solid. They transmit the full power of the engine through to the chassis, and you'll feel everything from it. Polyurethane mounts are similar to the stock rubber ones, but they offer a little more insulation between the engine and chassis than a solid mount would, while still being stiffer than a stock one. Were it my decision, I'd absolutely go with a poly mount over solid anyday on a street car.