I don't have an RO/DI system due to water availability on a well (long story), and given the hardness of our water I'd be changing cartridges way to frequently, instead I buy the Distilled water in 5 gallon jugs (water cooler jugs). it probably is more expensive, but I know what I am getting, the ppm of dissolved mineral solids is indicated on the bottle. the producers are in my city and I have worked for them in the past so I know their process, and testing frequency of the water. no issues so far. I did find that my PH and Alk were a little low, and am now wondering if that is due to the distillation process.
if you can't get an RO/DI system I would definitely recommend the distilled water as the way to go. as another post mentions, when producing distilled water they usually go through various steps, including distillation (due to wanting to maintain the distillers over longer periods of time, they aren't cheap).