Phil, found out a little bit about the lightweigh pulleys. In short, the lighter weight means that less engine energy is being used to drive the pulleys, therefore allowing more power to be put down by the wheels, thus giving a few horsepower/better engine response. This can be further improved by underdrive (smaller) pulleys which are even lighter and take far less power to drive. However, this can have negative effects as not as much energy is going to stuff such as lights, power steering etc. Personally would go for standard size but lightweight as get some benefit but without any risk.