What Jerry said.
Propylene gylcol is slightly less toxic than Ethylene glycol. But unless you dilute both with water, they don't perform well as a coolant. In concentrated form, they don't transfer heat nearly as well as either 50/50 solution or plain water. As far as coolant efficiency, water is about the best you can get, The problem with plain water is that it's VERY hard to pump below about 32F.
