i don't think distilled and deionized are the same. They might be though. Distilled is the best for water cooling as it is essentially 100% pure H20. Combined with water wetter it helps to reduce corrosion.
Actually,Deionized water is the one that is almost pure, not distilled, but there close.
"Distillation is a method where water is removed from the contaminants rather than trying to remove contaminants from the water. Distillation involves boiling water, capturing the steam, cooling it and condensing it back to liquid. Materials that have a higher boiling point than water will not rise with the steam. Most contaminates have a higher boiling point than water and therefore will not rise with the steam. Chlorine, however, is one substance that has a lower boiling point than water. It changes to chloroform gas and can be carried into the "treated water" with the steam."
after this is done there are still some minor impuritys to remove them Deionize it,
"The vast majority of dissolved impurities in modern water supplies are ions such as calcium, sodium, chlorides, etc. The deionization process removes ions from water via ion exchange. Positively charged ions (cations) and negatively charged ions (anions) are exchanged for hydrogen (H+) and hydroxyl (OH-) ions, respectively, due to the resin's greater affinity for other ions. The ion exchanges process occurs on the binding sites of the resin beads. Once depleted of exchange capacity, the resin bed is regenerated with concentrated acid and caustic which strips away accumulated ions through physical displacement, leaving hydrogen or hydroxyl ions in their place."
also deionized water dosent conduct electricity as well.
but as for deionized water in a water cooling system, I dont thnik it would make much of a difference, expecially since water wetter and/or anti-freeze would be added anyway..I would stay with distilled..here are the links that I got this info from.
I worked in a pharmacy for a couple of years, and deionized water was what we used for preparing titrated solutions (like Amoxicillin).The only reason we used it is that you need to eliminate all outside factors when mixing medicines for people. In a watercooling system, I don't think there'd be any advantage over distilled, such a level of purity would only be ruined by shooting it through the tubes/pump/etc..... I don't even know where you'd get it.