I agree with your first point, salt water is bloody annoying to work with. That and performing maintenance on a nuclear reactor to replace corroded elements would probably involve shutting the reactor down.

However, they should both cool equally well. When cooling hot objects with a cold fluid, the important characteristic of the fluid is heat capacity (how much heat the liquid has to absorb before raising in temperature), not freezing point. The addition of salts to water lowers the freezing temperature of the solution and thus takes longer to freeze. Adding small concentrations of salt shouldn't have a big impact on the heat capacity of your coolant.