i assume that its better because it doesnt have any minerals or anything dissolved in it. but you can buy special, pre-mixed fluid for the W/C kit.
EDIT: ahh, heres something
" All water is not created equal. You should use distilled water or de-ionized water. These aren’t as conductive as tap or bottled water. If water happens to spill out, then chances of saving your computer are greater with distilled water. You also need to add something to the water to keep organics from growing in it and to prevent corrosion. I’d recommend Wetter Water or Zerex racing fluid." -http://www.devhardware.com/c/a/Hardware-Guides/Water-Cooling-Basics/