I've been using distilled for years because I assumed there would be less corrosion. I wrote he guys at Royal Purple because I wanted to use their Purple Ice coolant additive and they had this to say:

"Using distilled water will increase the radiator's susceptibility to damage as distilled water is not electrochemically balanced; it is stripped of impurities and therefore ionically hungry, which will cause it to attack soft metals more aggressively. Drinking, spring, or softened are the recommended water types for use in a radiator, particularly in racing applications."

Doesn't make sense to me, what do you guys think?