Deionised water is much the same as distilled, except considerably cheaper. Pure water will pick up CO2 and various other things shortly after being placed in the system, so the difference between the two is likely to be slight.
Mixing it with ethylene glycol makes it a slightly worse coolant, but it wont support algae or freeze if things get too cold. Otherwise copper sulphate/pt nuke tend to kill algae, and a kill coil achieves much the same thing again. I prefer the latter since it's install and forget.
You can make arguments in favour of the more exciting ones based on it extending pump life, looking different, or in rare examples having a slightly higher specific heat capacity. For ten to fifteen quid a litre, relative to 80p for deionised + 3 or 4 to kill algae, got to be the latter. Especially if you think you'll go through a lot of coolant.