That's technically right. Distilled can be ionically imbalanced, but it often can get close to balanced just from being mixed with E/G antifreeze (the coolant becomes a donor).
If running straight water, then it's more of a problem. In such a case, the soft water is best (it is ionically much more balanced, having had Na ions added back into it as I recall).
The bottom line is that none of it will probably ever matter. I use distilled water myself. Were I going to run straight water with some additives for lubrication and rust inhibition, I would probably be a little more anal about it.
MHO.
Good luck.