The distilled water having metals thing is basically non-existent these days. Many years ago, some distilleries still used metal piping and the fear was that it could rust and affect the water. Pretty much every water distillery switched over to plastic many years ago, so distilled is equal to RO/DI in terms of quality for our tanks. The reason I say "basically non-existent" is because I obviously can't speak for every mom and pop distillery in the world, but any large reputable brand is definitely on plastic by now. I can say with 100% certainty that Poland Spring is completely pure and safe to use, as I've used it for the last 6+ years.
The preference of RO/DI is greatly tied to cost and ease of use these days. Being that I have a nano and live in an apartment, it's just more convenient for me to pick up a few gallons at the grocery store than setup a full water storage station.